Use File[] arrays for file/directory parameters

Maven 2.2.1 (perhaps other older Maven versions) seems unable to figure
out what type to pass for the directoryRoots and sourceFiles parameters.
It tries to pass Strings and blows up.
Avoid this by taking these parameters as File[] arrays.
Amend test following method signature change
No need to use File[] throughout

Use File[] for file parameters in the Mojo only. Use generic
collections as soon as we build the TaskContext.
Merge pull request #17 from dhurrell/files-as-arrays

Support older Maven versions - use File[] arrays for parameters