Release of first stable Python 3.x compatible version of MCStatTools

This version has the following features:

• brand new XML parsing which still parses counters for all generator methods though it is obvious fast MC productions will always have truncated/incomplete groups of xsection counters
• modernised argument parsing using argparse instead of optparse; also some flags and parameters are changed as documented in readme.md; improved command line argument validation
• functions in GaussStat module have possibility to access stat table repository information using web API (no longer relying only on direct filesystem access to statistics repository)

The code may still further development, especially exception handling in critical areas, and most likely a thorough clean-up of old and unused functions, but it can go into production as it is since MCStatTools would no longer work with Python 2.x.