Removed dependency of requests from okapi

Removed dependency of requests from okapi and added dependency to request_lib exceptions

@lorenzogil @gasull @haplo @rmariad 
It is better to keep the version number untouched until we decide to do a new release. After we merge this PR we will probably do a new release, but as a process, we should do these two changes in separate commits.

What you can actually do in this PR is update the Changelog here: https://github.com/redbeacon/okapi/blob/master/CHANGES.rst