Allow external session manager

[awarecan]

Feature Request

It will provide more flexibility if python-nest support external session manager. Proposed constructor change likes:

    def __init__(self, username=None, password=None,
                 user_agent=None,
                 access_token=None, access_token_cache_file=None,
                 local_time=False,
                 client_id=None, client_secret=None,
                 product_version=None,
                 session=None):