Fix grammar of licensing sentence, add IRC links

Also various formatting, wrapping and capitalization tweaks.
Add instructions on running the tests
Rename git_threads_ to git_libgit2_

This describes their purpose better, as we now initialize ssl and some
other global stuff in there. Calling the init function is not something
which has been optional for a while now.
Merge pull request #2637 from libgit2/cmn/global-init

Rename git_threads_ to git_libgit2_
Merge pull request #2653 from ethomson/contributing

Add test information to contributing guidelines