Fixes the problem where if there isn't anything to replace with, it replaces with "undefined"

https://github.com/yeoman/grunt-usemin/issues/128
Merge pull request #303 from jetpad/patch-1

Fixes the problem where if there isn't anything to replace it with, it replaces with "undefined"
Add support for revving src attribute of video and source tags
Added poster support for video tags
Address linting with video attributes
Added poster support for video tags

Conflicts:
	lib/fileprocessor.js
Merge pull request #254 from wbinnssmith/video-and-source-tag-support

Add support for revving src attribute of video and source tags
Use lodash more.
Gruntfile.js: add a test alias for default task.