Add an explict Count to IHeaderDictionary to resolve ambiguity.
#80 - Add CancellationToken to GetClientCertAsyc, GetFormAsync.
Make CancellationToken a default param.
Rename CancellationToken parameter.
Expand feature support for OWIN->K.
OWIN: Support dynamically creatable features.
Rename T to TFeature, Creator to Factory.