Add forms, advocacy campaigns and campaigns methods

[joe-irving]

Added functionality for:

• Forms
• Advocacy Campaigns
• Campaign Pages

I have added to the Readme and added tests - and would like to work through the rest of the API at some point but needed these for a project. Core things that are missing that might be an issue are:

• Submissions for forms
• Outreaches for Advocacy Campaigns

Is it okay to merge?

[anero]

Thanks for submitting this PR Joe! I think it looks mostly right, apart from the examples on README I think it would be good to handle hitting the rate limit on calls to all.
Docs are unclear on the response when hitting the rate limit but I imagine we'll get an exception from ActionNetworkRest::Response::RaiseError if Action Network returns a status code with error.

[anero]

It seems like you can easily go over the rate limit while using this method. May be worth adding some logic for retying with a slight delay if we reach it

[joe-irving]

Added to this method so it now limits the rate is calls list to every 0.25 secs max (as the docs mention). It also exponentially backs off if the 429 (Too Many Requests) response code is received. I think this should do it.

Worth noting this makes running bundle exec rake -t a little slower because it test the back off is working okay.