Import STL

This Pull Request adds support for importing STLs. 

STLs are note face-vertex meshes, so we need to generate topology data. This can be an expensive process for some meshes. For a large mesh (100k triangles) this can take about 3 minutes, while naive import (repeating vertices) takes under a second. It is possible this can be improved some way, but I need to do more research. As a consolation I added a `has_topology` flag to the `Mesh` composite type so we can get the best of both worlds for now. 

There were some cosmetic and functional changes I made to the code base as well. This PR enables the package to be tested from the REPL in 0.3+ with `Pkg.test("Meshes")`. I also added some test STL files to test the import.

This is a start of merging some of my code from [MeshSlicer.jl](https://github.com/sjkelly/MeshSlicer.jl) into this package.