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 page](http://slimerjs.org/faq.html) says SlimerJS embeds Firefox25, which means MediaRecorder API is available, so we need to figure out how to disable the permission prompt in SlimerJS.