Lots of upgrades & features

Please note that because this pull request is based on my previous pull request it includes those commits.  For a more accurate picture of this pull request, see https://github.com/econsultancy/activeform-rails/compare/a98fd5b...support-non-model-backed-forms

Please note this pull request addresses the following problems I encountered using this Gem:

* ActiveForm assumed a `master_model` was always defined.  This is not a good idea IMHO simply because form objects are very useful for validations or encapsulating view & form logic even if this does not directly correlate with an ActiveModel model.
* #new_record? was not available so various tests broke when I tried to integrate this gem
* I added alias_property so that you can map a field with one name to another, this is very useful when there are name conflicts.

Do tell me what you think, everything includes good test coverage and passes on Travis.