setPollFrequency(number) function added

Gpio.setPollFrequency() can be called to set frequency for the
fs.watchFile
Tidy up setPollFrequency, add test coverage and fix test that assumes callback will be second argument. Fixes #1
Update readme with setPollFrequency
Check modified time before emitting pin change event, as per API docs recommendation
Log debug message if read fails after watchFile triggers
Bump to v0.4.1