Add test cases for #97 - failed eager loading on parsed models

The ParsedPost model mutates its foreign key on `parse`, which causes
eager loading to fail to pair it to its parent. Non-eager loading (ie,
using #related instead of withRelated) is successful.
Parse eager-loaded models after pairing; fixes #97
Merge branch 'fix-#97' of github.com:benesch/bookshelf into benesch-fix97

* 'fix-#97' of github.com:benesch/bookshelf:
  Parse eager-loaded models after pairing; fixes #97
  Add test cases for #97 - failed eager loading on parsed models
test for morphTo parse, relation code cleanup