Fixes #928

[brycethomas]

The existing implementation of .gexf serialization/deserialization exhibited several issues with reading and writing the graph's mode attribute, leading to dynamic graphs always being interpreted as static graphs. This fix addresses these issues. I have removed the mode parameter that was being passed to GEXFWriter's constructor as the mode seems like an attribute that should be stored on the graph itself (which it now is) and write_gexf was not passing through a value for mode to the constructor anyway.

There were several cases where data attributes pertaining to the mode of the graph were being defined but not used, which I've replaced with instance variables.

[hagberg]

Looks good, just need to fix the failing test.

[brycethomas]

The fixes in the new commit (c74fef6) correct a small issue with the mode dictionary key not existing when the graph is constructed in networkx rather than read in from a .gexf file. I have also changed one of the previously failing unit tests slightly to accommodate the mode attributes existence in the graph dictionary.