Version 1.7.1 - new HTTP module; bug fixes; doc updates.
fix(identity):raise Internal Server Error for 500s

Currently pyrax expects a response body that can
be deserialised from identity, but in case of 500s
we would not receive a message body. This patch
attempts to fix that by adding an exception called
Internal Server Error and raises it if we receive
500s from identity.
Merge branch 'master' into working
fix(identity):raise Internal Server Error for 500s. GitHub #330