Incorporated the filename revving recipe

The deploy.subtask target now looks for a property called
filename_revving_target. If set within the file that is importing
wordpress-master.xml, it will be treated as a reference to a target
and invoked via phingcall.

The target will receive the current vale of export_dir. It should then
make one or more calls to subtask.rev-filename, each one specifing a
source file to be hashed and a directory path containing files that
may reference the source file.

The overall workflow looks like this:

1. Parent build.xml calls a deploy task defined in
   wordpress-master.xml (such as deploy.dev-to-stage)

2. deploy task invokes deploy.subtask

3. deploy.subtask invokes the target specifed by
   filename_revving_target

4. The target specified by filename_revving_target invokes
   subtask.rev-filename