Typo in principle_username argument

I think the new argument in the __init__ function in presto.py was meant to be `principal_username` not `principle_username`.

Might need to wait for a major version change as it's a breaking one. Or to add the `principal_username` argument, deprecate the old one and wait for the major version to remove it.

https://github.com/dropbox/PyHive/blob/bbf79418cfd2806fd9079893e70acc50a727a65d/pyhive/presto.py#L82