Add backend filter fold option to multi_backend

Adds an option that can be passed to multi_backend bucket folds. The
option {backend, [BACKEND_LIST]} will cause the fold to skip over any
backends not explicitly named in the backend list.

This enhancement addresses issue #1036.

This is my first Basho PR, so apologies in advance if I've neglected any
important details! Fingers crossed :)
Huh, weird. I didn't realize they could be anything other than atoms, but I guess other types should work just as well. I'm still learning the basics with Riak, but are bucket props all supposed to be binaries? From the type specs in riak_core_bucket_props, it looks like anything should be valid, and in the unit test I wrote it seems like atoms worked fine. Or, then again maybe I'm totally misinterpreting something....