Require python3 in shebang line

Encode the explicit requirement on python 3 in the shebang line for
script usage.
Check for nona and add command line argument

Checks for a nona executable and adds a command line argument to specify
an executable. The argument is required from the user in case nona
cannot be found on the path.
Let argparse print default values

Use a formatter for argparse to print default values instead of doing
this manually.
Replace system calls with subprocess

Replaces the os.system call for nona with a subprocess.check_call call
to get informed about errors when calling nona instead of silently
ignoring them before failing with missing files.
Merge pull request #8 from languitar/multires-improvements

Multires python script improvements