Extra dot in front of paths like 'path' and 'fallbackPath' are not necessary and may cause unexpected errors.

Hello,

the minified built version of pannellum.htm was not correct cause this space in the replace line was not replacing the corresponding line in src/pannellum.htm

        <link type="text/css" rel="Stylesheet" href="css/pannellum.css"/>

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.

Use a formatter for argparse to print default values instead of doing
this manually.

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.

Encode the explicit requirement on python 3 in the shebang line for
script usage.