Python 3 support

Support Python 3 while keeping Python 2 compatibility. The list of changes/commits (can be reviewed independently):

• Run futurize stage 1
• Replace execfile with importOptions
• Run futurize stage 2 (tweaked)
• Use inspect.signature for Python 3
• Replace math.log with just log in functors
• String/bytes compatibility with Python 3
• Flush printout of ApplicationOptions object
• Python 3 requires __hash__ when __eq__ is overridden
• Do not mutate MC checking categories
• Remove extraneous print statements
• Use None instead of empty string to signify default
• Remove unnecessarily high timeout from tests
• Python 3 compatibility for reading opts and pkl options

See LHCBPS-1858 Must be applied together with LHCb!2694 (merged)