Update ensureIndexes signature to return a Future instead of Unit

The ensureIndexes method on the Indexes trait was returning Unit, even
though the underlying calls to ReactiveMongo will almost always be
returning a Future.  Updated this to allow the Future to be returned.

This allows test code to await the index building repsonse, and gives
production code the option to handle errors that may occur during the
creation of the index.
Added documentation and a simple test for the ensureIndexes method.

Primarily to demonstrate that the type of the return value is implementation
dependent, and how futures can be combined in cases where there are multiple
indexes to be created.
Merge pull request #5 from hmrc/ensure-indexes-return-future

Update ensureIndexes signature to return a Future instead of Unit