Make describe easier to call

[rferg]

This is sometimes useful to call from the Rails console during development time to get the metadata for an sObject (e.g., required fields, relationship names, etc.). However, you have to call it by getting a reference to the sfdc_client, something like this:

model = Af::Campaign
ActiveForce.sfdc_client.describe(model.table_name)

Seems like it would be really easy to add as a class method on SObject, since sfdc_client and table_name are already available on that class. So it would instead look like this:

Af::Campaign.describe

Then we could add some convenience methods for filtering through that response for common cases, like checking if a field is nillable or getting a relationship name, etc.

Long-term, this might be useful to have for building more optimized queries that deal with relationships (e.g., "joins"). If we could cache this response on the class and then re-use that to build queries. Although I haven't really thought this through very much.