Merge pull request #1 from airbnb/master

Pull changes from airbnb rendr master
refactoring: separated the getModelForSpec method

getModelForSpec actually was a getModelOrCollectionForSpec as it was either
calling one or the other way. To make this clearer and improve the naming to
avoid confusion we separated the method into two separate methods
getModelForSpec and getCollectionForSpec
refactoring: extracted method that adds app to the options object
refactoring: removed unnecessary if

underscore already handles falsy values for defaults
created method that builds options by merging specified arguments with some
default options
Merge pull request #184 from lusini/pass-collection-options-consistently

Pass collection options consistently