Add support for the "Slow down cowboy" error raised when more than
200 requests are made in a 15 minute interval
Change rate limit exception name to better match the error text
Add a pre-fetch hook.

Allows user code to be executed before calls to the comicvine api

e.g. to implement rate limiting
Handle missing resources more gracefully.

Currently if any comicvine resource type is not modelled then the Types
class will raise an exception.

Add  a default UnknownResource class, and map resources to this by default
Pull missing types into master
Merge branch 'fetch-hook'