Review Bot has been heavily dependent on

pkg_resources.iter_entry_points for tool lookup. This is fine to use

for an initial scan, but we were calling it on every tool execution,

which was slow, and made it hard to write unit tests against anything

not provided by a package.
This change introduces a basic tools registry in

reviewbot.tools.base.registry. This handles tool discovery,

registration, unregistration, iteration, and fetching of tools.
Tools are only discovered on startup now, and we fetch directly from the

registry when executing one.
Unit tests were added to cover the new registry functions.

Unit tests pass for Python 2.x and 3.x.
Manually tested starting the worker, loading in the tools list into

Review Board, and running tools against new code.