Provide BestGems API

Some people hope to access to data of bestgems.org by a program.
So, I determin that providing BestGems API for powerusers.

API specification draft is [here](https://github.com/xmisao/bestgems.org/wiki/BestGems-API-v1-Specification).

I deployed this enhancement to production environment for testing.
Now, you can access BestGems API like following example.

`curl http://bestgems.org/api/v1/gems/x2ch/total_downloads.json`

Request for comments :)