Adding Spray Session directives and Session Authentication

Hello this is my first PR so please go easy on me :)

The following is a possible PR that includes new Session directives and a new Session Authentication context to be used with the authenticate directive (eg. remember-me feature on login). The implementation for: CookieJar was taken from the Play! Framework,  inspiration for the session directives were taken from: https://github.com/azeem/spray-cookiebaker and the Crypto functionality is also a stripped out version provided by the Play! Framework

At the moment because I'm not sure if this will go in, I've only included a single spec test to show proof of concept of "logging-in" a user based on session. If you think the change is good enough to make it in, I will go ahead and fill out all the remaining missing test cases.

Proposed commit message on squash:
```
= spray-routing spray-util spray-routing-tests: Added new session directive and session authentication 
```