Add option to clear the DNS cache on any edit or other DNS change

[2ndalpha]

Pre Mac OS 10.7, the command is:

dscacheutil -flushcache

For Mac OS 10.8 and 10.9. the command is:

sudo killall -HUP mDNSResponder

https://code.google.com/p/gmask/issues/detail?id=82

[TraderStf]

FYI, don't know if it is correct or required for Yosemite :

Clear local DNS cache

OSX 10.10
sudo discoveryutil mdnsflushcache
sudo discoveryutil udnsflushcaches

OSX 10.9
dscacheutil -flushcache; sudo killall -HUP mDNSResponder

OSX 10.7 – 10.8
sudo killall -HUP mDNSResponder

OSX 10.5 – 10.6
sudo dscacheutil -flushcache