version 0.7.1

• New script: sample-reads-randomly.py which does a single pass random
  subsample using reservoir sampling.
• the version number is now only stored in one place
• Makefile: new dist, cppcheck, pep8, and autopep8 targets for developers.
  VERSION is now set by versioneer and exported to C/C++ code.
• README switched from MarkDown to ReStructuredText format to clean up PyPI
  listing. Install count badge added.
• doc/: updates to how the scripts are called. Sphinx now pulls version
  number from versioneer. C/Python integration is now partially documented.
  Reference to bleeding-edge has been removed. Release instructions have been
  clarified and simplified.
• all python code in khmer/, scripts/, and tests/ should be PEP8 compliant now.
• khmer/_khmermodule.cc has gotten a once-over with cpychecker. Type errors
  were eliminated and the error checking has improved.
• Several fixes motivated by the results of a Coverity C/C++ scan.
• Tests that require greater than 0.5 gigabytes of memory are now annotated as
  being 'highmem' and be skipped by changing two lines in setup.cfg
• warnings about -Wstrict-prototypes will no longer appear
• contributors to this release are: @ctb, @mr-c and @cswelcher.