go-ao: libao bindings for Go

The API corresponds one to one with the C API, with the following exceptions:

- ao_play is a method of the Driver type (ao_driver), and has two variants:
  * Play8([]byte)
  * Play16([]int16)

- ao_close is a method of the Driver type

- ao_file_extension and ao_is_big_endian are not implemented.

Things have been renamed to conform to Go conventions.

For documentation see the C API documentation:

  http://xiph.org/ao/doc/