Add a basic .gitignore
Simplify the AudioPath name generation

Remove the duplicated "indexed" path name generation and just let the
index path types generate the name themself. This also make things
simpler for future type that might need some other index names.
Add dedicated outputs for the left, right and center buses

The bus outputs make it possible to use the samplers in setups using
an external mixers.
Properly handle error cases in findClosestBeat()
Add a primitive local BPM computation

To better cope with tracks having various speed change replace the
global BPM with a measurement of the BPM around the current play
position.