Support for Multi Task targets

Should still be backwards compatible with the html/css targets.
Note: Only patched the `usemin` task, not the `useminPrepare` task.
Fixed CSS url: path matching

usemin:css was not correctly replacing revved resources. Will write up a test later, but here is the bug fix.
Implements mainConfigFile in require.js config
Merge pull request #61 from sleeper/fix_54

Implements mainConfigFile in require.js config (fix #54)
Merge pull request #59 from cbas/patch-2

Fixed CSS url: path matching
Merge pull request #58 from cbas/patch-1

Support for Multi Task targets