Add support for multiple PowerMates

[dkordik]

Adds support for multiple PowerMates by passing an index to the PowerMate constructor:

var powermate1 = new PowerMate(0);
var powermate2 = new PowerMate(1);

When omitted, falls back to index 0.

var powermate = new PowerMate();

[sandeepmistry]

Thanks!

Do you have time to update the unit tests? If not, I can do it on the weekend.

[sandeepmistry]

Thanks, released a new version 0.0.2 as well.