Rewrote and extended Python code.

Major changes include:
* compatibility with both ST2 as well as ST3
* the plugin is now less eager to switch to themes that the user doesn't have
  installed
* new command to select a random preset (so theme+colour scheme combination) in
  addition to just colour scheme
* friendlier display text for randomly selected colour scheme (just the name, so
  withouth the path and extension)
Document default settings file.
Expanded command palette entries.
Shamelessly clean up formatting.
Expand readme.
Remove license.txt (contents are now in readme).
Adjust a message slightly.