wacom: add USB tablet support

To be honest, it's more like "be less discriminating against USB tablets".
USB tablets usually get autodetected, device name is not necessary and defaulting to a serial touchscreen is a clear discrimination.
Unconditionally remapping buttons is generally not a good idea either.
Old defaults transformed into examples.
Remove .conf extension from acpid config files

This should fix #51
Merge pull request #64 from oxij/master

Fix wacom and cherry-pick acpid fix by surr