Defined the sparql::writer binding serialization interface.
Imported the xml_writer.h header file from the librdf++ project.
Implemented the xml_writer#finish_element(full) method.
Finished the SPARQL XML writer implementation.
Ensured that <cstdio> functions are used with namespace qualification.
Defined some useful fields for sparql::writer::implementation.
Implemented the CSV and TSV writers for SPARQL query results.

See: http://www.w3.org/TR/sparql11-results-csv-tsv/
Implemented the sparql::writer#write_binding() method.
Fixed bugs and finished the SPARQL JSON writer implementation.
Ensured that ./configure checks for the open_memstream() function.

See: http://pubs.opengroup.org/onlinepubs/9699919799/functions/open_memstream.html
Improved the unit test harness and added fixture data files.
Implemented the sparql::writer#omit_binding() method.
Defined the sparql::format structure type.
Implemented the sparql::format::supported() static method.
Implemented the sparql::format::find_by_content_type() static method.
Refactored sparql::writer implementation instantiation.
Ensured that sparql::writer#begin_result() resets the binding counter.