Add mget on index type

Hi there!

This commit adds an mget method on the index type model, which then allows you to pass an array of IDs to mget and get back those documents.

It's a bit easier to use and cleaner than the mget that's on server, since it's more specific and requires you to pass less metadata about indexes and types. As a result, I feel it's a nice addition to the stretcher API.

Please let me know if you have any questions or concerns!

Thanks,

Dan