Added aliasDirectory option and corresponding tests.

Adds support for aliasing multiple (sets of) files using the options described at http://gruntjs.com/configuring-tasks#building-the-files-object-dynamically. Note that expand option is set to true by default so the user doesn't have to do it. I muddled my way through the tests so make sure they look OK. If everything looks kosher I will add a README update to the PR before you merge.