Enable maven plugin to recurse subdirectories for schema to code generation

[joelittlejohn]

Originally raised by @matallen:

The Problem
I wanted to parse multiple schemas where there were common element (and therefore class names) between them. the generation prefixes the classes with underscores which will be difficult to understand where the classes are intended to be used.

The Solution
I added an "includeSubdirectories" option in the maven pluging and the Mojo simply checks and subdirectories of the specified sourceDirectory for schemas (and adds the folder name to the package for easy identification of the class).

[joelittlejohn]

Merged changes from @matallen's fork.