Delaunay triangles with md5 hash and tests

Following [your suggestions](https://github.com/jonnor/imgflo/pull/52#issuecomment-51224307) Jon:

- Better name: delaunay_triangles
- Creates a md5 hash for the seed and then get some `substr's` to generate slightly different random numbers
- Always generates a md5 hash, even If no seed is given (maybe we can generate a random md5 hash each time no seed is given)
- First tests for no seed and for a default seed ("foobar")