update __init__ for Session to get argbitrary keywords for use with ssl

hi.
I've noticed I cannot access initialization of self.protocol in Session(), if I want to pass various ssl related params (certificates), so to be able to do this - I'm adding **kwargs.
I could have explicitly specified what protocol params I want to pass, but this would be of lower future compatiblity.
This seems to break nothing and seems to not require any additional tests.
Tell me what you think.