Skeletor

New post-processing methods:
- `skeletor.post.smooth` to smooth the skeleton
- `skeletor.post.despike` remove spikes from skeleton
- `skeletor.post.remove_bristles` to remove single-node bristles from the skeleton

This release mainly improves `skeletor.skeletonize.by_wavefront` but it also
adds a new method to get a graph representation of skeletons:
`Skeleton.get_graph`.

This is a small release that adds a single method to the `Skeleton` class to
quickly save results as
[SWC](http://www.neuronland.org/NLMorphologyConverter/MorphologyFormats/SWC/Spec.html)
file:

Python
>>> skel.to_swc('skeleton.swc')

| ------------------------------------------------------ | ---------------------------------------------------- |
| `skeletor.skeletonize(mesh, method='vertex_clusters')` | `skeletor.skeletonize.by_vertex_clusters(mesh)` |
| `skeletor.skeletonize(mesh, method='edge_collapse')` | `skeletor.skeletonize.by_edge_collapse(mesh)` |


New Features
- new skeletonization methods `skeletor.skeletonize.by_tangent_ball`,
`skeletor.skeletonize.by_wavefront` and `skeletor.skeletonize.by_teasar`
- all skeletonization functions return a `Skeleton` object that bundles the
results (SWC, nodes/vertices, edges, etc.) and allows for quick visualization
- SWC tables are now strictly conforming to the format (continuous node IDs,
parents always listed before their childs, etc)
- `Skeleton` results contain a mesh to skeleton mapping as `.mesh_map` property
- added an example mesh: to load it use `skeletor.example_mesh()`
- `skeletor` now has proper tests and a simple [documentation](https://navis-org.github.io/skeletor/)

Removals
- removed `drop_disconnected` parameter from all skeletonization functions -
pleases either use `fix_mesh` to remove small disconnected pieces from the
mesh or drop those in postprocessing
- `output` parameter has been removed from all skeletonization functions as the
output is now always a `Skeleton`


Bugfixes
- plenty