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 find this approach very hackish. `GraphMLWriter` and `GraphMLWriterLxml` should select by themselves the `Element` class suited to their own purposes instead of relying on a global name being overridden externally.