commands.py: explicitly tag integer arguments as such

--reference-set-size and --reference-set-timeframe should be declared as
integer types to optparse. If this is not done, they are parsed as
string objects, which triggers errors such as:

    $ afew -vv --update-reference --reference-set-timeframe=360
    Traceback (most recent call last):
      File "...snip.../.local/bin/afew", line 9, in <module>
        load_entry_point('afew==0.0.0', 'console_scripts', 'afew')()
      File
    "...snip.../afew-0.0.0-py2.7.egg/afew/commands.py",
    line 164, in main
        time.time() - options.reference_set_timeframe * 24 * 60 * 60,
    TypeError: unsupported operand type(s) for -: 'float' and 'str'