Added DataMapper::Adapters.adapter_name(const_name)
Added DataMapper::Adapters.adapter_name(const_name)

This is a semipublic api addition that is used at
least in dm-transactions for now. It seems like a
natural counterpart to the #adapter_class method
that looks up the adapter class based on the adapter
name (as returned by this newly added method).

engineyard/dm-core,datamapper/dm-core,tpitale/dm-core,alloy/dm-core,tillsc/dm-core,ar-dm/ardm-core