remove dependency on activesupport 3

[karmajunkie]

Remove this dependency so that the gem can be used with any ruby project, including Rails 2.x.

[statianzo]

You may find aequitas useful for validations to replace ActiveModel's. It has integration with Virtus.

[statianzo]

Using aequitas or any other validation library is shouldn't be a hard dependency. For those who do already use activemodel/support, being able to reuse that functionality would be good instead of pulling in a library with similar-but-not-the-same features.

[karmajunkie]

Agree wrt it not being a hard dependency; in fact I'm not sure I see any reason to have it integrated into the gem at all, other than a mention in the README.