Added convenience functions for arrays of JSON/Models

You can now use [MTLJSONAdapter modelsOfClass: fromJSONArray: to convert an array of JSON dictionaries to models and then [MTLJSONAdapter JSONArrayForModels] to get a JSON array of dictionaries from a array of models.  Closes #176.
A bit of documentation clean up
Code cleanup in response to PR comments
Fixed a few more Tab/whitespace issues
Clean up formatting, variable names.
Remove .idea files from project
Nil checks and error returns

JSONArrayFromModels now has an assert to check if models is nil.  modelsOfClass:fromJSONArray:error: now returns an error if the JSONArray is nil or its not an NSArray
Removed redundant nil check

In the MTLJSONAdapter there was a nil check that is no longer required as it returns an error now if nil
Remove whitespace from top of modelsOfClass
Fixed indentation issues
Remove appcode .idea folder
Merge pull request #299 from RobotsAndPencils/master

Convenience functions for converting arrays of models to models and to JSON.