diesel:
* allow to implement the join_dsl for multiple joins between the same
  table. All join related traits are now abstract over the foreign key!!
* Implement JoinTo for already joined tables to allow joins containing
  multiple tables. Currently it's not possible to join two tables
  multiple times.
* remove select_column_workaround! and add generic implementations for
  this
* remove join_through, because it's now possible to build joins
  containing more than two tables

diesel_codegen:
* add an optional parameter to the has_many and belongs_to annotation
  naming the foreign key column
* improve name building in codegen (use the Inflector crate for this. We
  don't need to implement this on our own)