Fixed #112 - more_like_this raises InvalidIndexError for unindexed instance.

This doesn't happen if silently_fail is True, in which case returns
an empty result. Added test to cover both cases.
Merge pull request #121 from jorgecarleitao/issue112

Fixed #112 - more_like_this raises InvalidIndexError for unindexed instance.

This doesn't happen if silently_fail is True, in which case it returns
an empty result. Added test to cover both cases.