Fixed Ruby warnings.

Moved class definitions out of before hooks.
Add contribution guidelines
Lead with references to discussion platforms
Should link to the mailing list properly
README searches for "mailing list" should work
Nicer indenting is nicer
Merge pull request #215 from celluloid/contribution_guidelines

Add CONTRIBUTING.md for reminders for issues
Move the Contributing file to the root
Add the irc:// protocol to the URI
Fix minor contributing copy
Move some Thread monkey-patches to the Celluloid module

Use a subclass for Threads