Nurbs-python

* Bug fix release

* PyPI description update (requires version bump)
* Minor updates and bug fixes

* Reorganize and update Matplotlib visualization component, `VisMPL`

Please note that updates to `VisMPL` component might cause a small API break on the visualization module, and therefore it is marked as experimental from now on. Updates to visualization component do not affect NURBS evaluation components.

Currently, `VisMPL` has the following classes:

* `VisCurve2D` for 2D curves
* `VisCurve3D` for 3D curves
* `VisSurfWireframe` for surfaces (uses `plot_wireframe` function)
* `VisSurfTriangle` for surfaces (uses `plot_trisurf` function)
* `VisSurfScatter` for surfaces (uses 3D `scatter` function)

* Added checks to knot vector assignments
* Added 'tangents', 'normals' and 'binormals' methods to Curve classes
* Added 'tangents' and 'normals' methods to Surface classes
* A new example added to documentation

* `normal` method of the `Surface` type classes has been updated. It now returns a list containing 2 elements. First one is the starting point (or origin) of the normal vector and the second one is the normal vector itself.
* Added `normal` and `binormal` methods to the `Curve` and `Curve2D` type classes
* Minor fixes to the inline documentation
* Visualization figure updates

* Bug fixes in `Surface` classes
* Added tests
* AppVeyor and Travis-CI integration
* Documentation updates
* Style updates and code reformatting after pylint