get_signature_request_list() is lacking parameters  #75

The  get_signature_request_list() method only takes page and page_size as arguments, which prevents the use of some of the optional request parameters. account_id and query should be added as arguments as well.

ie:
def get_signature_request_list(self, page=1, page_size=None, account_id=None, query=None):

Currently the only workaround is to use the requests library and query that way.