Refactor to make more import friendly

[chm0dx]

Some minor changes to make things more import friendly. The experience should be nearly identical when invoked from the command line but improved when imported.

• Prints relegated to main() with results returned from associated functions rather than printed directly
• FireProx.error() raises a custom FireProxException, handled and printed with try/except in main()
• Requirement for a command argument restricted to main()
• FireProx constructor no longer requires an argparse.Namespace argument so that FireProx() can be called directly without jumping through hoops

Thanks!