Fixed belongs_to name in m:m join model to use the singularized relationship name
Fixed belongs_to name in m:m join model to use the singularized relationship name

* This allows you to use a different relationship name from the target
  model.  Previously in this case it was assumed the relationship would
  always be the underscored target model name, which is not always true.

[#406 state:resolved]

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