move local isrc checks to a function

The goal is to stop having code in global namespace.
move global dupliate check to function
move "global" code to __main__
create find_backend()
make constants all caps
move automatic backend selection to gather_options
move browser default to global constant
we don't need versions for outdated backends
fix more backend parameters

We only use the backend as parameters for functions where the backend is
important.
For error messages etc. we actually have options.backend.
fix default_browser reference
differentiate between local and MB discs/tracks
always print used backend
make disc not a global object
debug -> options.debug
fix missing "errors"
add/remove newlines in output
Merge branch 'move_code'

Conflicts:
	isrcsubmit.py