WIP proxy support

re: chfoo/wpull#41

[ci skip]
Basic HTTP proxy support.

RE: chfoo/wpull#41
Support proxy authentication.

Closes chfoo/wpull#41
Merge branch 'issue/41-proxy_support' into develop

Conflicts:
	doc/changelog.rst
	wpull/builder.py
builder: Capture warnings if INFO or lower.
Don't handle StopIteration.
Add --ignore-fatal-errors option.