Release version 2.0.0

This plugin has been around for a while and in production in many Redmine
installations. The current 0.2.0 version didn't do this justice.

Bumping all the way to 2.x to signal the compatibility with Redmines 2.x line.
Going forward major version numbers should follow Redmines major versions.
Gemify

This should allow you to install the plugin as a gem, simply by adding it to
your Gemfile.local before bundling.