Bug fix in nlevel.min_tips() and improved compliance with scikit-bio 0.3.0.

* fixed bug in nlevel.min_tips() which was causing an exception
* improved compliance with upcoming scikit-bio v0.3.0 by making use of the new TreeNode read() and write() functions. There is still some work to be done to bring tax2tree into compliance with  scikit-bio v0.3.0 deprecation notices.
* Minor improvements to parsing of consensus-map files.