Check for sticky dates.

If a previous build did check out a specific "revision" (timestamp)
we can't update the directory with cvs update (unless we use -A).
Upload the CVS/Entries file from the slave and check for any sticky
dates.

Entries look like this:

/Filename/1.1/Fri May 17 23:20:00 2013//D2013.10.08.11.20.33
D

Just make sure the last field of each line does not start with D
(and skip the final line)
Fix the tests, add a test for sticky dates.
Refactored CVS step copy method.

Pulled out CVS/Entries parsing into
"_cvsEntriesContainStickyDates(entries)". Tests should still pass.
On Python 2.5 and 2.7 install txrequests

Required for several tests:

buildbot.test.unit.test_steps_http.TestHTTPStep.test_404
buildbot.test.unit.test_steps_http.TestHTTPStep.test_POST
buildbot.test.unit.test_steps_http.TestHTTPStep.test_basic
buildbot.test.unit.test_steps_http.TestHTTPStep.test_header
Fixed tests broken by b40ac18
Added test for _cvsEntriesContainStickyDates
Release notes
Merge branch 'travis_install_txrequests' of git://github.com/vrutsky/buildbot
Merge branch 'cvs-check-sticky-dates' of git://github.com/jpommerening/buildbot