Changes to support latest Mac OS X and CentOS/Ubuntu distributions + bug fix

Hi, firstly thanks for the awesome code :).  Really useful.

Something must have changed in the ipconfig command recently because your code broke when I tested it on CentOS 6.3 and the latest Ubuntu.  It worked on OS X straight away though.  I've made a fix to solve the problem...

The code looks for 'HWAddr' instead of 'ether ' when it runs ipconfig.  It also converts to lowercase when comparing the actual MAC vs. the one that the program wants to set the interface to.  There was also a tiny bug in an if-statement for checking if the system is Linux or not.

Would you like to pull these changes?

Thanks again, this was a great help.