CORS Headers 

As a preliminary to CORS support -  https://github.com/spray/spray/issues/297 I want to contribute model for necessary headers- Access-Control-* and Origin-*

Probably this version is not yet final, but if you can please comment on:
1. Parsing method names. It is currently in spray-can I did it separately here
2. Access-Control-*-Headers - as header names I used Strings to have all headers - both defined and custom. Maybe it is not an ideal way to do that - I appreciate any comments.