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