Try to open on other ports if default port is not available

[Varunram]

A normal issue is that people often find port 30303 to be used by other programs and stuff. If this is the case, we should simply move on and try to open gmc via port 30304 and 30305 and so on (maybe lets cap the number of ports to be tried at 5/10). This would ensure that gmc starts without any issue (although this doesn't solve collision with the Ethereum port).

[Varunram]

Related: https://github.com/Musicoin/desktop-interface/issues/8