Hooked up libxml2 to the SPARQL XML writer.
Defined the public methods for sparql::writer.
Ensured that sparql::writer is movable but not copyable.
Fleshed out the XML writer implementation.
Implemented the sparql::writer#write_head() method.
Implemented the sparql::writer#write_link() method.
Added std::string convenience overloads for sparql::writer methods.
Ensured that the sparql::writer destructor is declared noexcept.
Implemented the sparql::writer#write_variables() method.