Remove active attr.

[eloyesp]

We are currently using a patched version of ActiveAttr, but I think that it was a bad design decision, I'm almost sure that we should remove this dependency and use ActiveModel using these instructions instead.

I think that this is important as it will simplify the gem and will make it easier to maintain and understand.

@eloyesp @ayanbule First of all thanks for your work on this! Yesterday I started work on a gem called salesforce_mapper that active_force seems to fulfill most of the requirements for already. I'm going to evaluate active_force a bit more today to see if it fits our needs, and if so I might be very interested in working on a PR to swap out active_attr for active_model. Based on what's in here so far, do you think this would be a significant amount of effort?

[eloyesp]

As active_attr uses ActiveModel it seems that it should not be so hard to do the switch. Also, it would be great as we are monkeypatching active_attr to make the `dirty` part work. Thanks!