Add retry support to RequestsContext.

commit dfdf21ce5c9aa2db8259f0579519ad5bbbe17ed5
Author: Joshua Cohen <jcohen@twitter.com>
Date:   Fri Nov 7 15:11:25 2014 -0800

    Add retry support to RequestsContext.

 pex/http.py        |  35 +++++++++++++++----
 tests/test_http.py | 100 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 2 files changed, 127 insertions(+), 8 deletions(-)

Testing Done:
$ tox

Reviewed at https://rbcommons.com/s/twitter/r/1303/