Cleanup, support for specifying one or more file/wildcard following the bitrate, and support for spaces in filenames/paths

Running with only the **bitrate** as an argument will behave the same as before, except now files that have filenames containing spaces will work too.

You can now also specify a list of files or wildcards to convert, which will be processed instead of `*.mp4`, ie:
 * `mp4towebm 8M file1.mp4 file2.mp4 file3.mpg` (we're assuming `file3.mpg` is an incorrectly named **mp4**)
 * `mp4towebm 8M new_*.mp4 to_process*.mp4`
 * `find . -type f -iname "*.mp4" -exec mp4towebm 8M '{}' \;`

Cheers!