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.
Related to https://github.com/aspnet/Mvc/issues/743. Should we have a Get cancellationToken on the base action result and the interface, and just call it? (separate PR if that the way). For now it can just return CancellationToken.None