Add support for manually managing the RestDocumentationContext

In 1.0, the reliance on JUnit was more widespread than it should have
been. It should have been isolated to the JUnit TestRule implementation,
RestDocumentation. Unfortunately, RestDocumentation was also an argument
to MockMvcRestDocumentation.documentationConfiguration which made it
impossible to use Spring REST Docs without having JUnit on the
classpath.

This commit introduces JUnitRestDocumentation and
ManualRestDocumentation. The format is a direct replacement for
RestDocumentation which has been reworked to delegate to
JUnitRestDocumentation. The latter allows manual management of the
RestDocumentationContext, primarily for use with TestNG.

A new interface, RestDocumentationContextProvider, has been introduced.
It is implemented by RestDocumentation, JUnitRestDocumentation and
ManualRestDocumentation.
MockMvcRestDocumentation.documentationConfiguration has been overridden
to also accept a RestDocumentationContextProvider. The method that
accepts a RestDocumentation has been deprecated, as has
RestDocumentation itself.

The documentation has been updated to encourage the use of
JUnitRestDocumentation and a sample illustrating the use of Spring REST
Docs with TestNG has been added.

Closes gh-171

samples/testng/src/docs/asciidoc/index.adoc

@@ -0,0 +1,21 @@
+= Spring REST Docs TestNG Sample
+Andy Wilkinson;
+:doctype: book
+:icons: font
+:source-highlighter: highlightjs
+
+This sample application demonstrates how to use Spring REST Docs with TestNG.
+`SampleTestNgApplication` makes a call to a very simple service and produces three
+documentation snippets.
+
+One showing how to make a request using cURL:
+
+include::{snippets}/sample/curl-request.adoc[]
+
+One showing the HTTP request:
+
+include::{snippets}/sample/http-request.adoc[]
+
+And one showing the HTTP response:
+
+include::{snippets}/sample/http-response.adoc[]