use tuple modules instead of parameterized modules

Erlang R16, coming soon, will do away with parameterized modules (see Issue
4 under http://www.erlang.org/news/35 for details). Change the
parameterized modules in riak_search to use tuple modules instead, since
they will continue to be supported in R16 and beyond. These changes are
backward compatible.
Merge pull request #138 from basho/sbv-drop-param-mods

use tuple modules instead of parameterized modules