Improve cython skiplist

[lsbardel]

The current cython skiplist implementation performs approximately 20 times faster on insertion than the pure python implementation. You can check this by running the benchmark test:

python runtests.py -t bench skiplist

There is plenty of room for improvement however. For example one could replace node lists with numpy arrays and switching off bound check with the cython decorator:

@cython.boundscheck(False)