allow to change interface/ip from GUI and with commandline with '-i' …

[Nightreaver]

…option

added check to validate local ip address
added .idea/ gitignore (PyCharm)

[kakaroto]

On closer inspection, I don't like the name of 'interface' in the command line options, because I would then do ./SWProxy -i eth0 or -i wlan0, etc.. that's what an interface is technically (in linux speak at least). so I would keep it as ip address, but make the usage/help specify that it's the listening address to use.
Also, if you make it modifiable in the GUI, it needs a 'Reset' button in case someone changes it by mistake

Anyways, thinking about it... the easiest would probably be to simply bind 0.0.0.0 (ANY interface), the IP address here is only shown to the user so he/she knows what to put in their phone's proxy configuration. I don't really see the point of binding a specific interface, instead of all possible interfaces. Maybe this has been discussed already in #38, but I don't have time right now to read the backlog there.