Fix grammar of licensing sentence, add IRC links

Also various formatting, wrapping and capitalization tweaks.
Add instructions on running the tests
Changed GIT_REMOTE_DOWNLOAD_TAGS_ALL to behave like git 1.9.0
Merge pull request #2673 from swisspol/2672

Fixed GIT_REMOTE_DOWNLOAD_TAGS_ALL to behave like git 1.9.0+
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
Fixed git2.h not including threads.h anymore
Fix GCD typo
Merge pull request #2708 from phatblat/pr/threading-doc

Fix typo in THREADING.md
Merge pull request #2707 from swisspol/fix_include

Fixed git2.h not including threads.h anymore
Plug possible leak in the openssl locks
Merge branch 'openssl-lock-free'
Fixed active_refspecs field not initialized on new git_remote objects

When creating a new remote, contrary to loading one from disk,
active_refspecs was not populated. This means that if using the new
remote to push, git_push_update_tips() will be a no-op since it
checks the refspecs passed during the push against the base ones
i.e. active_refspecs. And therefore the local refs won't be created
or updated after the push operation.