Add function versions of choose and control:

control n, amp: 0.5

choose [1,2,3,4]
Use standard keys for sample groups and add new helper fns
Print out message if sample had to be loaded into memory
Teach load_samples to handle an array as an arg
Add new functions: sample_names and sample_groups:

* sample_names will return *all* sample names as symbols, unless a group
  is specified and it will then only return the sample names in that
  group

* sample_groups returns a list of all sample groups as symbols
Unify sample output format
Add new example - ambient:

* takes advantage of new sample discovery functions