Add DeprecationWarnings to inform users of plans

After a long discussion in IRC and on several issues, the developers of
requests have decided to remove specific functions from requests.utils
in version 3.0.0. To give users ample time to prepare for this, we've
added DeprecationWarnings long in advance. See also the planning of this
in issue #2266.

My only concern with this is that these DeprecationWarnings are on by default
in Python 2.6 and off everywhere else. Should we disable them for 2.6? It's a
not insignificant portion of our user base that may be affected by this.