Update to ck 0.3.3 and introduce git subtree for ck vendoring

First commit contains the changes necessary for ck 0.3.3.  Would appreciate some peer review here!
* In upstream, ck_ring changed quite a bit.  ck_ring_init() doesn't touch the buffer, and it was removed from ck_ring_t.  Instead, a new opaque ck_ring_buffer_t must be separately initialized and is passed in to each enqueue() and dequeue().  https://github.com/sbahra/ck/commit/3edb523da5f6c9dcea41879ceb6643dcc7bde305 

The rest is a switch to using git subtree to vendorize ck.  For a good overview, http://blogs.atlassian.com/2013/05/alternatives-to-git-submodule-git-subtree/.  IMO, this is the least smelly way to vendor something, carrying the full repo is fine, and this gives the full power of git for maintenance of the vendor branch.