Add container methods, constantly resize window

Empty clontainers can now be constructed in either a flexible or
fixed-size form. Fixed-size containers will scroll to fit their
content. In order to avoid wrapping issues, the window now no longer
resizes horizontally, but now constantly resizes itself to fit its
contents.
Add a audio class for playing sounds from URLs

The audio class loads a compatible sound file from a URL, using an HTML
audio element to handle playing, pausing and looping the audio.