Nurbs-python

* Fixed an issue which causes figure display problems during rendering curves and surfaces in `Multi`-type classes
* Coding improvements in `utilities` module
* Added more tests, increasing the code coverage to 47%

* New feature: Exporting plots as image files
* Documentation: Added [exporting plots](http://nurbs-python.readthedocs.io/en/latest/visualization_export.html) section with code examples
* Documentation: Updated [load-save](http://nurbs-python.readthedocs.io/en/latest/load_save.html) section with some code examples

* Bug fix: The sample generator function was not considering the starting value

* `exchange` module updates
* Documentation updates

* Several bug fixes and compatibility updates
* Documentation updates

This is the first **beta version** of NURBS-Python v4.x series.

* Updated `NURBS` class control points / weights getters and setters
* Rearranged / updated some functions and properties to clean up the `BSpline` and `NURBS` class structure
* Control points reading and CSV export functionality are moved to `exchange` module
* Added more tests
* Updated examples
* Evaluation algorithms speed improvements

* Added [Plotly](https://plot.ly/python/) visualization module
* Updated documentation and added more details
* Added new export file formats
* Added B-Spline to NURBS converters

4.x update might break `NURBS` class API a little bit due to the updates in getters and setters but the fix would be very easy: Please see the `ctrlptsw` and `ctrlpts` properties for details.