v0.6.0

• Completely rewrote Parsers in a Pythonic, non-multiprocessed way
• Remove lxml from requirements. pronto now looks for any
  available xml parser among (in that order): lxml,
  xml.etree.cElementTree, xml.etree.ElementTree.
• Added a second OwlXML parser using a TargetCollector instead of
  building a tree, that is longer but less memory-hungry
• Added a 'parser' kwarg to Ontology to force usage of a parser.
• Removed Parser being imported with 'from pronto import *'
• Implemented partial Unicode sandwich, at least in Relationship.
  Complete use of unicode internals is a v1.0.0 requirement.
• Added a try/except block to use enums if available to represent
  the current section of the files parsers are exploring
• Ensured PyPy support
• Add full support in Python3 and partial support in Python2 of
  gzipped files parsing.
• Refactored tests. Now all tests are run through the unittest
  interface, even doctest.
• Cleaned dead code