Allow for request timeouts to be specified

By default requests has no timeout, this is risky when using it from a server backend. Similar to the verify that got exposed recently, this adds a timeout parameter, which defaults to the requests default of None, to refresh_token and fetch_token.
Merge branch 'patch-1' of https://github.com/RR2DO2/requests-oauthlib into RR2DO2-patch-1

Conflicts:
	requests_oauthlib/oauth2_session.py
Merge branch 'RR2DO2-patch-1'