Bumps version to 0.18a1.
http. Refactors ConnectionPool/HostConnectionPool interaction.

Moves semaphore release outside finally clause to maybe fix worker hang
problem (chfoo/wpull#27). ConnectionPool now calls put on
HostConnectionPool instead of sharing a queue. This allows for assertion
check when the HostConnectionPool is stopped and closed.
changelog: Updates latest to 0.17.1.
Bumps version to 0.17.1.

Merge branch 'develop'

Conflicts:
	wpull/version.py