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 Mochiweb to use
tuple modules instead, since they will continue to be supported in R16 and
beyond. These changes are backward compatible, so current Mochiweb
applications should require only recompilation to continue working.