FileOptions -> SourceOptions to work with String paths instead of Files

We check for existence of the sources and (as before) resolve multiple
resources on the classpath if a path is not a File. In addition supports
Spring pseudo-URL prefixes as well as normal URLs as source locations.

In addition sources can now be specified as a directory (searched
recursively by default), or a resource pattern (e.g. app/**/*.groovy).

Fixes gh-207