Subclass IOError for HTTPServiceError.

Asked `requests` to do the same kennethreitz/requests#1532.
Subclass HTTPServiceError from AssertionError

@jslang convinced me this is better.

Demands should raise an AssertionError while errors from requests are IO.
Add response to HTTPServiceError
Fix typo
Merge pull request #14 from yola/doug/except

Subclass HTTPServiceError from AssertionError