Update graphml to care for a number of issues. (#2515)

* Added graphmlxml class

* Allow graphml to read Cytoscape files with incomplete headers

Fixes #2197

* graphml pep8 changes

* Add a test and some docstring for graphml json unicode treatment

addresses #1880

* Change read_graphml and parse_graphml docstring

Fixes #1398

* Update LXML writer to include attributes, work with py36 etc.

Set LXML as default write_graphml if present.
Fixes #448  using code from #1219

* Attempt to get travis to load lxml.

* Correct testing both xml and lxml writers in graphml

* skip docstring test

.travis.yml

@@ -114,7 +114,7 @@ install:
       if [[ "${TRAVIS_PYTHON_VERSION}" =~ "2.7" ]]; then
         pip install pygraphviz;
       fi;
-      pip install pydot scikits.sparse;
+      pip install pydot scikits.sparse lxml;
     fi
   - if [[ "${PYTHON_VM}" != ipy ]]; then
       pip install --upgrade nose coverage coveralls;