Merge OslcClient and OslcRestClient

[berezovskyi]

• Ensure DefaultRequestHeaders overwritten #19 and DefaultRequestHeaders overwritten in OslcClient #204 are solved properly. I am thinking of OslcRequestParams that we pre-set in the library, allow the user to set in the ctor, or to override on a per-request basis (mainly, to request/specify certain types, e.g. request only RDF/XML for root services or POST RDF/XML-ABBREV in some special cases)
• Allow access of both unmarshalled POCOs as well as access to the raw response graph.
• Allow the client to be set to accumulate the responses in a single graph - useful for the initial discovery phase.