Added graphmlxml class

It is an attempt to solve issue #448.

Note that this is a preliminar CL, and probably is necessary to polish some edges:
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.

Note: It's a resubmission of https://github.com/networkx/networkx/pull/1074
I suppose that Travis' 50-minute time limit is for individual jobs, not the entire build. Now that the time limit is long enough for building NumPy and SciPy from source, adding lxml would not be too big a problem.