Applies some code comment fixes.
Adds Device.LEDState() method. it returns the current,
global LED state for the given device.
Modifies bitset methods to accept signed int parameters.
This makes its use a bit more practical. Additionally, it
implements a Bitset.Len() method, which returns the number
of bits in the set.

Traversing a bitset to test for values no longer requires
us to reference specific evdev constants, related to the bitset.
We can simply use set.Len() instead, to determine the
iteration bounds. E.g.:

    set := device.KeyState()
    for i := 0; i < evdev.KeyMax; i++ {

Becomes:

   set := device.KeyState()
   for i := 0; i < set.Len(); i++ {
Modifies example programs to reflect changes to Bitset type.