assetDir, gulp-rev

Add assetDir option.
Add gulp-rev integration (to emulate the functionality of grunt-usemin:prepare.

I am submitting this as a working example. These are almost the last features needed to allow gulp-usemin to replace grunt-usemin for the yeoman angular-fullstack generator. 

The only other feature I can think of is the alternate path format of 
```({.tmp, app}).``` I believe if the file is found in the first directory, the second is ignored. This issue isnt something I really need but it might be nice to integrate. 

Thank you for your work on this!