Changes related to switching to NetworkX 2.0

Adaptation of the bg library to work properly with the newest major version of the NetworkX library, which is the foundation of bg. The only things that might break with this:

• BreakpointGraph.bg.edges and BreakpointGraph.bg.nodes now return view like object, rather than an indexable list, so if you've used internal bg field in the BreakpointGraph data structure for direct access to the stored data, make sure to adapt to the new view like approach.
• following the change in NetworkX, all edge/node data attributes are now located in the data["attr_dict"] dictionary instead of the data dictionary. This is a simple one more layer fix that can be done in every case, where internal data for the edges/nodes is utilized directly, rather than using the respective attributes (e.g., multicolor, data, etc) on the BGVertex and BGEdge.

1.8

Polish of the bg library:

• added full support for the Python 2.7, and now library is Python2/3 compatible
• restored integration with the travis ci and coveralls
• restored documentation integrations with the readthedocs
• multiple bug fixes