Allow for recursively searching `groovy.classpath` directory entries.

[RyanHecht]

Right now, the groovy.classpath configuration item accepts both folders and individual .jar files. However, in the case of folders, it expects a flat directory filled with .jar's, with no nesting.

There are many circumstances in which .jar's may be in a more structured directory tree, such as in a local Maven repository. It would be nice to be able to specify /home/user/.m2/repository/**, for example, to include the entire local Maven repository in the classpath.

Since entries ending in * are currently assumed to be flat directories, I propose entries ending in ** be considered directories that ought to be recursively searched.