Add LICENSE.txt file and make it clear in the README file that we
are Apache 2.0

Fixes gh-1952

Fixes gh-1948

Closes gh-1919

Previously, only the http.mappers.json-sort-keys property was applied
by JacksonAutoConfiguration. This commit updates it to also apply the
http.mappers.json-pretty-print property as well.

See #1919

Closes gh-1936

Closes gh-1906

Closes gh-1905

Fixes gh-1944

Previously, for a non-async response with a successful status (< 400),
ErrorPageFilter would always call flushBuffer. This triggers an
exception in Tomcat if the client has closed the connection before the
response has been fully sent. In this case, Tomcat treats the response
as successful and commits it before control returns to the filter.

This commit updates ErrorPageFilter to only perform the flush if the
response has not already been committed, leaving any further flushing
that may be necessary to be handled by the servlet container.

Fixes gh-1938

Update ConfigurationPropertiesReportEndpoint to find
@ConfigurationProperties using `context.findAnnotationOnBean` rather
than `AnnotationUtils.findAnnotation`. This will correctly find the
annotation even if the bean is an interface based proxy.

Fixes gh-1927

Switch from external docbook XSLT to those shipped as part of the
Maven plugin.

Fixes gh-1942

Relax the servlet version check in Maven integration tests.

See gh-1797

Fixes gh-1797

Fixes gh-1874

Closes gh-1785

Closes gh-1893

The exception message for a connection timeout has been updated to
include the timeout period. The tests for the sample have been updated
accordingly.

Closes gh-1884

Closes gh-1883

Closes gh-1882

Closes gh-1881

Closes gh-1880

Closes gh-1879

Closes gh-1878

Closes gh-1877

Closes gh-1876

Closes gh-1875

Fixes gh-1712

Update MultipartAutoConfiguration to detect any MultipartResolver beans
rather than just StandardServletMultipartResolvers.

Fixes gh-1857

Closes gh-1863

Closes gh-1862

Closes gh-1739

If a Gradle build is using the Spring Boot Gradle plugin's support for
declaring dependencies without versions then they will be unable to
publish Maven artifacts from the build as the Gradle-generated pom
will fail to validate. This is because Gradle doesn't apply the
Boot-provided dependency versions to the dependencies in the generated
pom.

This can be addressed by configuring Gradle to generate a pom that
either imports spring-boot-dependencies into its dependency management
or that uses spring-boot-starter-parent as its parent. This commit
updates the documentation to document the need for this configuration
and to provide examples of how to do so.

Closes gh-1806

Traditionally, a @ServerEndpoint-annotated bean is found by a servlet
container initialiser, however Boot does not run servlet container
initialisers when an embedded container is being used. To be able to use
@ServerEndpoint in a Boot app that uses embedded Tomcat a
ServerEndpointExporter bean must be declared.

This commit updates the documentation to describe this requirement and
also updates the WebSockets sample to illustrate the use of
ServerEndpointExporter. The version of Spring Framework has been updated
to 4.0.8.BUILD-SNAPSHOT. This picks up the fix for SPR-12340.

Closes gh-1722

Closes gh-1861