Fix for #70, options.columns is not propagated to relations

Fix for #70, omitting options.columns when eager
loading relations.

Internal refactoring of the Collection object to match
the Model, adding the _handleResponse and _handleEager
methods. Test cleanup to match the mocha-as-promised
syntax universally.
documentation updates for 0.5.3
release 0.5.3
* index.html - minor fix, correcting the version number in docs/changelog

is 0.5.3 not 0.5.2
Merge pull request #76 from gildo/patch-1

* index.html - minor fix
merging upstream into morphToMany