new method for press_keys

This allows for multiple key presses a bit easier than currently. Eg.
k.press_keys(['Command','shift','3'])

I added this as I was getting a little frustrated writing multiple lines to handle deal with presses and releases when doing keyboard shortcuts. 
@SavinaRoja correct me if I'm wrong but I can't get these examples to work.. It does raise a question though that whether there needs to be some changes to Mac.py (or Win.py) to make this the same..