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
Move `libxml2-dev libxslt1-dev` to [line 75](https://github.com/networkx/networkx/pull/1219/files#diff-354f30a63fb0907d4ad57269548329e3R75). Move `lxml` to [line 93](https://github.com/networkx/networkx/pull/1219/files#diff-354f30a63fb0907d4ad57269548329e3R93) and remove/modify [line 91](https://github.com/networkx/networkx/pull/1219/files#diff-354f30a63fb0907d4ad57269548329e3R91).