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.
Short answer : Both the scenarios that you mentioned will match. 
Long answer: This would work because if you look inside the implementation of CanWriteResult in ```OutputFormatter``` it uses the following check. 
```c#
SupportedMediaTypes.FirstOrDefault(supportedMediaType => supportedMediaType.IsSubsetOf(contentType));
```
This means since supportedMediaType is a subset of the accept header that is passed in, both the scenarios the formatter will be chosen.