getPath (aka deepGet)

I've implemented a deep property getter based on my original proposal in #28.

Some differences with this version:

* I renamed it `getPath` to match `updatePath` and `setPath`.
* It only accepts an array of keys as the selector, not a string with dot notation. This is also to match `updatePath` and `setPath`.

@michaelficarra had noted some similarities between #28 and https://github.com/documentcloud/underscore/pull/1106. With the changes mentioned above, there is less overlap in the proposals.

I've attempted to follow the existing code and testing style, but let me know if something needs to be changed.