Bumps version to 0.7a1.
Fixes chunked transfer encoding field match.
Moves RobotsTxtSubsession into robotstxt module.
Adds URLItem. Reduces tight coupling with Engine and ProcessorSessions.
Rewrites robots.txt concern into a mixin for WebProcessorSession.
Moves the URLItem status setting concern out of Engine.
Bundles robotexclusionrulesparser because it is not hosted in PyPI.

Resolves pip insecurity warning.
Updates readme with links and third-party code notice.
Fixes robots.txt handling logic flow.
Bumps version to 0.7.

Merge branch 'develop'

Conflicts:
	wpull/version.py