API should implement SPARQL 1.1 Protocol specification

[leipert]

HTTP API should follow SPARQL 1.1 Protocol specification.

So HTTP GET should allow the following query parameters:

• query (exactly 1)
• default-graph-uri (0 or more)
• named-graph-uri (0 or more)

Furthermore it should use the Accept header instead of a output query parameter (content negotiation):
Accept: text/turtle, application/rdf+xml

[nareike]

Updated to support POST and GET requests as well as content negotiation.