improvements to make_otu_heatmap.py

Made the following changes to make_otu_heatmap.py:

- fixed bug where long labels were cut off
- generated figure makes better use of whitespace in general
- added new options ``-g``/``--imagetype``, ``--dpi``, ``--width``, and ``--height``
- removed ``--output_dir`` optional option and replaced it with the required option ``--output_fp`` because the script only ever generated a single file
- updated code to use scikit-bio for distance calculations, fixed `VisibleDeprecationWarning` that numpy was raising
- cleaned up the plotting code, fixed import namespace issues
- cleaned up some of the script's help text

Fixes #1571. @gregcaporaso can you please review?