Add configuration options for gwt.persistentunitcache and gwt.persistentunitcachedir

It would be nice to have configuration options for these 2 properties in compile and run goals (maybe others need them also).

As noted in PersistentUnitCache javadoc:

• gwt.persistentunitcache : enables the persistent cache (eventually will be default)
• gwt.persistentunitcachedir=<dir> : sets or overrides the cache directory

The configured <dir> will contain the directory gwt-unitCache where the cached items will go.
By default it will be the directory that contains the directory specified with -war option ( See it here )
So I think that it must be changed to put into target directory or subdirectory.