add tests for Async function call
make getCover async
add include (memory) for std::shared_ptr
done reformatting, remove unused code
Tweaks for merge:

* The node custom is to name sync methods Sync but to have async methods
  be the normal name - just following node core style here. This does
  incur some pwn since it'll break projects but rather eat the
  refactoring debt now than later.
* One class of errors wasn't getting errors provided - (err, res) was
  (null, null). Modified this to return a descriptive error, and added
  test assertions to make sure other errors are provided as well.
Merge pull request #68 from mapbox/GetAsyncCover

Get async cover