Add external Lua module support

As the sandbox cookbooks were being developed a grammar section was being
created.  However, the only way to reuse the grammar was to cut and paste it
into a plugin... it was time for external Lua modules to allow for
standardization and reuse of the code.
Use specific revisions instead of branch names for external git repos.
Whoops, forgot changing the source repo also changes the import
path, and neglected to 'make clean' when testing. :P
Merge pull request #533 from mozilla-services/sandbox_modules

Add external Lua module support