Update requirements.txt to require gevent 0.13.1

This properly reflects the new standard set in
f685ae38e5ab590d68c5895883aeabd47c43dfbf, on discovering 0.13.0
probably never worked for anyone anyway after setting up tox.ini to
exercise this..

Conflicts:
	requirements.txt
Rename PipeViwerRateLimitFilter to PipeViewerRateLimitFilter

This was a typo.
Run tests on Travis CI
Add Timothée Peignier as a contributor
While transferring WAL ensure concurrent greenlets can clean up

There have been some reports that WAL-E can leave around temporary
files pathologically often.

This is because when an error is propagated to the main greenlet,
there's a good chance that it will exit the process before allowing
the other concurrent greenlets to clean up: instead, those greenlets
are abruptly terminated, and in doing with temporary files are leaked.

This bug probably exists elsewhere in WAL-E (e.g. base backups), but
the introduction of parallel WAL upload has rendered its effect
uniquely noticeable to date, probably because so many of
start-up-shut-down cycles of WAL-E are performed in a short period of
time, and each such cycle is an opportunity to leak these files given
an error condition.