GraphML serialization shall by default be ordered and have an improved ordering

To ease readability and regression tests based on GraphML files (especially used for loading Graphs into Gephi) I suggest to

a) order (or normalize) the GraphML by default (e.g. from Gremlin console)

b) improve the ordering if numbers are involved, oppose to String only:

Currently, in the reference test file for normalized GraphML "tinkerpop-classic-normalized.xml", the ordering of edge IDs goes like

10  
11
12
7
8
9
instead of 

7
8
9
10   
11
12

It goes even jumpier if hundred or thousand of edges are involved.

I am going to provide a GitHub pull request.