Implement audio capture API

Please visit the following url to run the test MIDlet:
`http://localhost:8000/index.html?jars=tests/tests.jar&midletClassName=org.mozilla.AudioRecorder`

you can set the pref `media.navigator.permission.disabled` to `true` to disable the permission prompt.
oops, there is a typo, when `AudioRecorder` is stopped on the other side, it will sent a `stop` message, anyway, let me rename it to `close`.