Implement a data source based on SPARQL protocol only

[mhgrove]

this would actually subsume all current implementations, at least for those which support sparql update, since we would not need a database specific implementation for each vendor. though that might be preferable in some cases to provide more optimal bindings, the ability to use a different (non-http) protocol, etc. Need to resolve #76 first though so we can even use SPARQL update syntax