Refactoring to have all metrics operate on lists.
Sort spikes if necessary.
Do not use sampling_rate of spike trains.

It is per default 1.0 Hz which is completely bogus.
Some refactoring.

Allows to use some distance functions with non-symmetric kernels while
still being efficient with symmetric kernels.
Documentation.
Refactoring schreiber_similarity.

Potentially increasing performance for symmetric kernels.