ADD: IPython notebook with a tutorial on how to use pydstk.

ENH: LinearDS.cluster does no longer compute pairwise dist., i.e.,
     the user is responsible for providing the distance matrix.
     Made all required adjustments to dtcluster.py.
ADD: LICENSE.md
ADD: Testing data for clustering (might move to MIDAS, though).
STYLE: Remove .DS_Store file from repository

[ci skip]