Add a command to produce a self-contained executable JAR for a CLI app

A new command, jar, has been added to the CLI. The command can be
used to create a self-contained executable JAR file from a CLI app.

Basic usage is:

spring jar <jar-name> <source-files>

For example:

spring jar my-app.jar *.groovy

The resulting jar will contain the classes generated by compiling the
source files, all of the application's dependencies, and entries
on the application's classpath.

By default a CLI application has the current working directory on
its classpath. This can be overridden using the --classpath option.
Any file that is referenced directly by the classpath is always
included in the jar. Any file that is found a result of being
contained within a directory that is on the classpath is subject to
filtering to determine whether or not it should be included. The
default includes are public/**, static/**, resources/**,
META-INF/**, *. The default excludes are .*, repository/**, build/**,
target/**. To be included in the jar, a file must match one of the
includes and none of the excludes. The filters can be overridden using
the --include and --exclude options.

Closes #241

spring-boot-cli/README.md

@@ -198,3 +198,18 @@ $ . ~/.gvm/springboot/current/bash_completion.d/spring
 $ spring <HIT TAB HERE>
 clean    -d       debug    help     run      test     version
 ```
+
+## Packaging Your Application
+
+You can use the `jar` command to package your application into a
+self-contained executable jar file. For example:
+
+```
+$ spring jar my-app.jar *.groovy
+```
+
+The resulting jar will containe the classes produced by compiling
+the application and all of the application's dependencies such that
+it can then be run using `java -jar`. The jar file will also contain
+entries from the application's classpath. See the output of
+`spring help jar` for more information.