Nngt

Further optimization of library support with direct inheritance of library graph objects by `Graph` and its subclasses (this means that `networkx`, `igraph` and `graph-tool` functions can now be used directly on `Graph` instances).
The library now fully supports all python versions from 2.7 to 3.5 and continuous integration through Travis is on its way.
New graph generators (`fixed_degree`, `distance_rule`) have also been added, as well as analysis functions to characterize the activity of the networks modeled with NEST.
Eventually, basic graph IO has been implemented.

Planned improvements for next release:
- support of all main graph formats for IO
- full support of node and edge attributes
- more complete tests for Travis

Here comes the 2nd release for my favorite debuggers ;)

This is a first step for the NNGT package: it's going to be tested at that point, so though it's not ready yet, let's call it the first official non-released version... yeah!