Use ReadTimeout as base timeout exception for the requests client

[sjaensch]

This fixes issues with code that catches requests.exceptions.ReadTimeout, which we broke by using requests.exceptions.Timeout as base class.

[coveralls]

Coverage increased (+0.003%) to 98.582% when pulling 95f28d3 on sjaensch:use-requests-readtimeout into c524bce on Yelp:master.

[ngaya-ll]

What about requests.exceptions.ConnectTimeout?

[macisamuele]

@ngaya-ll : connection errors are not integrated yet because we didn't find a reliable way of testing those type of errors with fido client; but definitely our plan is to "generalize" also those type of errors (#346)