Add vendored libmongoc

Compiled remotely with

  $ vulcan build -v -c "make && mkdir /tmp/mongo && INSTALL_INCLUDE_PATH=/tmp/mongo/include INSTALL_LIBRARY_PATH=/tmp/mongo/lib make install" -p /tmp/mongo

and then copied into the vendor/ directory.
Add parsing of mongodb:// URIs

The Mongo C driver doesn't do this (I think to avoid a glibc
dependency and/or rolling its own regex engine), so I ripped off the
tests and implementation from the Ruby one. I didn't do any of the
option handling though.

There's a test function that inserts a document at startup to confirm
everything's working.