Added class GraphMLWriteLxml to support incremental generation.

It is an attempt to solve issue #448.

Note that this is a preliminar CL to polish some edges, as at the moment:
- A new class was introduced and just in case lxml is available it uses it.
- There are some hacks, as to be able to reuse the existing code.
- The method generate_graphml is still based on the old writer.
- it lacks a dedicated .travis configuration (one with lxml)