Fix installation error on Python 3.

See http://pypm-free.activestate.com/3.3/linux-x86_64/pool/m/mu/murl-0.4_linux-x86_64_3.3_1.pypm.d/log
for the log.
Cosmetic changes.
Bump version to 0.5.1.
Delete .env.
Normalize whitespace.
Fix markup.
Update version info in docs.
Switch to the default RTD theme.
Cleanup docs/conf.py.
Add changelog.
Update hacking howto.