Vendor libmaxminddb

For #1.

Just hacked the basics together to get things working. Traveling right now.

* [x] Using 0.5.3 since thats what I was using from homebrew. But it looks like the latest is now 1.0.3. We should probably switch to that. At least this gem can normalize the library version everyone is using.
* [x] Should a tar just be checked in or is the 100 loose files okay?
* [x] Whats the conventional place to "install" the libmaxminddb lib here?
* [x] `rake compile` leaves a ton of untracked files in the working directory
* [x] Is there anyway to avoid the additional libmaxminddb object and statically build out a single geoip2_compat object?
* [ ] Should we get CI setup on this thing?

cc @dbussink @brianmario
This version is duplicated a few times. Think it should be a ruby const somehow?