Fixes examples/keys code to have a slightly more efficient
conditional expression.
Adds code comments to AbsInfo structure.
Fixes Device type goroutine synchronization after Device.Close
is called. It now uses a sync.WaitGroup to wait until the two
read/write poll routines are finished.

Refactors the Repeat function signatures.

Adds functions to query supported absolute axes and their
descriptors.
Adds examples/axes program. It demonstrates how to
query for absolute axis information.