1.7.0: drop support for Python 2.6

This is minor version bump since it drops support for Python 2.6

Version 1.5.4

Improved PEP-8 conformance, change logging to use logger, file opening (with), error handling cleanup, and removal of sys.exit in the library.

Version 1.5.3

Single bugfix that addresses a test failure on test_sha1_tagfile.

Version 1.5.2

Added SHA1 support to tagmanifest.