Remove existing mongo-c-driver directory

It'd have conflicted with the submodule.
Add back mongo-c-driver as submodule, lock to v0.6

Slightly better dependency management, easier to update, etc.
Update README to explain submodule initialization

To help transition users to the new way.
Switch to relative URL for submodule

This'll let people use whatever protocol (git, ssh, http, etc.) they
want when checking out the project and the submodule will automatically
use the same. See [this blog post][].

[this blog post]: http://blog.tremily.us/posts/Relative_submodules/
Initial code needed to output json.
First working prototype.
Added support for serializing objects.
Merge remote-tracking branch 'origin/feature/use_submodule_for_driver'

Conflicts:
	README
Updated the extension to work with mongo c driver v0.8.
Merge commit '2ac3c01' into json-type
Removed some worthless code.
Updated mongo c driver to version 0.8.1.
Changed print_json to use StringInfo.
Added string escaping for json.
Code style changes.
Added basic pgregress tests.
Merge branch 'pgregress' into json-type+tests
Added regression test for JSON datatype.

Also added code for outputing BSON dates to "MongoDB Extended JSON" [1]

[1] http://docs.mongodb.org/manual/reference/mongodb-extended-json/
Code cleanup.
Merge commit 'c632ac480e6fffc79c32845a34488574fc5bc685'

Conflicts:
	Makefile
	README.md
	mongo_fdw.c
	mongo_query.c