Adding support for content negotiation.

This change consists of :
1. Conneg based on request headers, supports the following 3 scenarios:
	a. ContentType property on ObjectResult set to null or is empty.
	b. ContentType property on ObjectResult set to a single content type.
	c. ContentType property on ObjectResult set to multiple content types.

2. Parsing Helpers, comparers and extensions for comparing various http headers.
3. Tests.

Open workitems:
1. Remodel JsonResult and ContentResult to be a derivation of ObjectResult.
2. Add support for acceptcharset headers.
3. Produces Content Attribute.
4. Sample for adding a result filter for adding a custom formatter to the collection.
We need a way to expose the algorithm to enable scenarios where users would want to con-neg for `Accept-Encoding` header(ex: `Accept-Encoding: gzip, deflate;q=0.8`) in case we do not support this out of the box...i.e a user should be able to run the algorithm to find out the encoding and do the compression himself...

Currently in Web API we do not provide this and so wondering if we can fix this...