variety of changes based on feedback

- changed back to direct register writes instead of read-modify-write
operations (probably faster)
- removed cli() and sei() from the PIT ISRs
- changed period specification from float (seconds) to integer
(microseconds)
- in light of above, changed the range check (invalid for value of 0 or
value greater than overflow condition, which is based on the bus clock
speed)
- invalid period now returns false instead of silent adjustment
- updated readme to reflect these changes
- updated example sketch to reflect these changes