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.
@yishaigalatzer : are you sure..I am referring to `gzip` and `deflate` compression mechanisms driven by `Accept-Encoding` header and not Charset...I say this because this PR is mixing bunch of terms which got me confusing too...

Also I do not want it to be tied to a formatter as a user might want compression to happen on any kind of response content: object going through formatter, a file stream etc.