Implemented a default constructor for rdf::blank_node.

The default constructor auto-generates the blank node label in the
pattern _:gN, where N is an atomic monotonically increasing integer.
Ensured that the Raptor reader generates blank node labels using the
rdf::blank_node default constructor instead defaulting to _:genidN.
Ensured that ./configure checks for the fmemopen() function.

See: http://pubs.opengroup.org/onlinepubs/9699919799/functions/fmemopen.html
Enhanced the rdf::blank_node default constructor for ID uniqueness.

The default constructor auto-generates the blank node label in the
pattern _:gNM, where N is the current microsecond-precision time
since the (Unix) epoch and M is a small (pseudo-)random integer.
Reformatted the <rdf++/triple.h> and <rdf++/quad.h> headers.
Implemented the rdf::triple default constructor and destructor.
Implemented the rdf::quad default constructor and destructor.
Implemented the rdf::triple#reset() method.
Implemented the rdf::quad#reset() method.
Added an rdf::reader constructor to support content type guessing.
Ensured that libraptor2 headers are referenced correctly, such that
it's possible to install the dependency in a non-standard location.
Removed some superfluous class fields from the Raptor reader.
Added support for content type guessing in the Raptor reader.
Ensured that Raptor reader will permit the base URI to be omitted.
Added support for the new official N-Triples/N-Quads MIME types.

See: http://www.w3.org/TR/n-triples/#n-triples-mediatype
See: http://www.w3.org/TR/n-quads/#sec-mediatype