added units
tests to specify measurement units passed
fixed round_timeframe for

There was a special with 23:00 hours. If you tried to round that with the old
function, you'll get dt.hour = 24, which breaks the datetime object because it
only takes ints between 0 and 23. Now the function uses dateime.timedelta to
increase the datetime by 1 hour if it's multiple of 3.
updated README with support to measurement units