Http Abstractions for parsing Http headers needed for conneg

Moved from http abstractions.
Well, MeadiaTypeHeaderValue is supposed to be representing Media-Type which never contains the quality factor. 
The subclass here is more of a convenience class which is used to represent an accept-header which has a media type + accept parameters ( which includes quality). 
BTW, these are throw away abstractions, as we are bound to change them once httpabstractions has its own.