Add submodules for rebar deps

The current dependency management solution is hard
to use when you might be tracking changes outside
of the official release cycle. This adds a tiny
bit of sanity back (submodules are barely better
but they are better).

Eventually, some of these dependencies should be
moved back to rebar dependencies since they are
simply mirrors of external projects. Others, like
the couchdb:couch module should probably be merged
back into the top-level repo. There is almost zero
value in having this split out as it is now.

I've also used a special URL scheme for the
submodules to allow easy selection of mirrors.
You can easily get the apache mirror by using
an insteadOf= entry to your gitconfig. For
example, I use something like:

  [url "https://github.com/strmpnk/couchdb-"]
    insteadOf = couchdb:

While it isn't an open ended URL rewriting tool
it does make it easy to create private clones of
all (34!!!) couchdb repositories.