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
Not in particular, but I've had had troubles in the past installing lxml from pypi. Also, as it usually involves compiling the sources it could be slower. In any case I would need to first install libxml2, and libxslt, before running pip install lxml.

For pypy I did not want to add an extra testing path as the whole process is already takin more than an hour, which is pretty close to Travis' limits.

I would also remove the cpython from the allowed failures.