Implementation of #81 - better $key interaction

* Added support for $bucket
* Added ListKeysFromIndex to perform correct key listing behavior from
$bucket index
* Documented GetObjects and GetObjectIds in RiakMapReduceResultPhase
* Clarified AllKeys documentation in RiakIndex
* Modified AllKeys to use $bucket instead of range comparison
* Corrected ToBinaryKey ?: operation so $key doesn't become $key_bin
* Added unit tests for new functionality