- added audio, video, and youtube reST directives; updated example3 to use these.
- initial 'black' theme (currently same as default)
- changed colors from 'default'
- hide context slides by default
- add flowplayer based "flashvideo" directive for playing videos with flowplayer
- set vertical height of images to 90%
- properly parse rst options of form ":optname: optval"
Merge code that adds new multimedia directives

audio, video, and youtube directives have been added, as well as a 'black'
theme.