added feature issue #135 interface binding

Added "-nic Default_Gateway" optional flag to specify gateway.

How to use in windows. Check "ipconfig" and see which all network interfaces are open. Use "ping -S IP hostname" to check its validity.
If you pass its gateway in '-nic' flag then source IPs would be different. Check it by accessing "http://whatismyipaddress.com/" (You may create proxy, set it in browser and check).

Known issues:
     I have seen some performance issues and sometimes proxy to server connection breaks. Apart from that some dongle/USB connection driver softwares set default gateway to "0.0.0.0" or similar. If this is the case then this gateway will be used for all purpose(I didnt use such connections). Task is to find a combination (Like LAN, WiFi, 2 or 3 Mobile connections through USB) that works. For me I got upto 5 different IPs (LAN, WiFi, 3 USB Connections).
Rather than introducing this Constants class, I would prefer to see this as an option on HttpProxyServerBootstrap, analogous to address, port, etc.