Add @options back into serializers for passing custom options

This is to maintain the functionality of render :json as noted in the docs

This functionality used to exist in v0.8.1. This adds the ability to
pass other options that will be usable in the serializer via the
options accessor.

This works by adding an attr_accessor for options so it is available
and is set by the remaining options in the provided options hash during
initialization.

Fix #446

Please let me know what you think! 

Cheers 
-Jason