Feature request: Themes on a separate file

I've moved the themes to a separate file, I also reorganized the powerline-separators.el file.
Other changes (didn't change any actual code):
  - removed "(require 'cl)" -> you're not using any CL function and from emacs 24 onwards it's cl-lib.
  - removed the "(message "powerlne loaded")" -> redudant, if emacs can't load the package you will know.
  - custom powerline-default-separator wasn't including the box separator, it is now.
  - enhanced some functions descriptions.

Possible bug:
  - shouldn't line 139 of the new powerline-separators.el be ",height" instead of "height"?