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.
This is hard to test reliably, but could you add a basic test that uses the API in a simple way just to make sure at least we don't throw exceptions?