Resampled buffer

Added `buffer.adaptative`, which is like `buffer` but reads the buffer at varying speed in order to keep the buffer at decent filling. For instance,
````
sleep(delay=1.5,s)
````
simulates a source which is 1.5x slower than realtime (thus catchuping quickly). The following script however accomodates quite well of it
````
s = playlist("~/Music")
s = sleeper(delay=1.2,s)
s = mksafe(s)
s = buffer.adaptative(s)
s = mksafe(s)
out(s)
````
@dbaelde @toots I have some minor stuff to fix but could you please review already it would help much. In particular, I am no so sure I am filling and reading from frames exactly correctly... Thanks!