Allow JSONAPI unsuffixed associations keys

As shown here :
https://github.com/json-api/json-api/blob/gh-pages/format/index.md#resource-relationships-
Auto-include url_helpers for Rails-apps. Fixes #563
Fix path where test-app is getting configured
Unsuffixed association keys tests
Unsuffixed association keys doc
Cleaner unsuffixed association keys tests
Merge pull request #639 from gauthier-delacroix/Unsuffixed-default-associations-keys

Allow JSONAPI unsuffixed associations keys
Support Rails 4.2.0 with AMS 0.9

This patch is the same fixes as #655 and #663 for 0-9-stable branch.
Merge pull request #674 from tricknotes/support-rails4.2-for-ams0.9

Support Rails 4.2.0 with AMS 0.9
Add serialization_options

Prior to 0.9, @options was used to see the options passed to to_json. For some reason, with 0.9 the options were no longer passed and were entirely inaccessible within a Serializer.

This restores the access by placing it in an attribute named serializer_options.
Fix serialization_options for array serialization
Make options param optional
Make sure serialization_options is never nil
Test to cover serialization_options
Merge pull request #666 from jastkand/include_url_helpers

Auto-include url_helpers for Rails-apps. Fixes #563
Merge pull request #679 from Ahalogy/serializer-options

Serializer options