Feature/commands

[ghalestrilo]

This branch creates a commands module to isolate commands that the user can issue. Here's a quick list of the ones implemented so far:

reset      
set        <*args>
debug      <key>
generate   <length, unit='beats'>
start      
stop       
load       <filename, length=None, unit='bars'>
save       <name>
buffer     <num>
event      <ev>
instrument <addr, index, *inst>
play       <pitch>
quit       
exit       
kill       
end