Add support for population filter within a request URL 

This PR adds the ability to selectively populate model relationships via the request URL. To make this work, a new blacklisted criteria option, `populate`, has been added. Everything works as it normally would except now you can also use `/model?populate=[child1,child2]`.

I was unable to find any bugs/feature requests open for this but it was mentioned in [issuecomment-39363108](https://github.com/balderdashy/sails/issues/124#issuecomment-39363108) of #124 and I had previously brought it up in [stackoverflow/23643870](http://stackoverflow.com/questions/23643870).