DDF-877 - Fixed an internal backbone-association's error caused by dot notation in json attribute names.

-json output containing attribute names with "." in their names were causing issues with the "_getAttr" method in Backbone.Associations.  When dot notation is used with a Model.set() or a Model.get(), it tries to do sub-object path based on the dot notation used (which is not what we want)
-Changing the Backbone.Associations.SEPARATOR from "." (the default) to "~" fixes the issue.  Sub-Selecting can still be done, but you will need to use the ~ character instead.
-The subObject pathing can be explained at http://dhruvaray.github.io/backbone-associations/specify-associations.html#sa-getsetop
Merge pull request #31 from McLucas/feature/DDF-877-association-fix