Add distance matrix script that uses MUMmer to compute distance between bins (fasta files)

- Adds some simple dir utility functions to the concoct python module i.e. ``rm -rf`` and ``mkdir -p`` in ``concoct.utils.dir_utils``
- Adds script that runs MUMmer's dnadiff on given fasta files. Then computes a distance matrix as ``1 - AvgIdentity if min(AlignedBases) >= min_coverage else 1`` of 1-to-1 alignments. 
- Has a test for all functions
- Uses multiprocessing for parallelization on a single node