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.
Well, I was in two minds, while this was part of http abstractions it didn't make sense because figuring out if a header was a subset of another header was a helper at best and should not have been exposed as a first class method on the abstractions. Now that it is part of mvc and that too a temporary thing. I think its ok to merge. 
@rynowak  the same thing goes for the MediaTypeRange property.