Docstring fixes, Raise Exception, KeyError: 'x-security-token', Pandas DataFrame output, nosetests unit tests

Hello,

this PR fixes docstrings
it raises exception when an error occured
if also fixes KeyError: 'x-security-token'
Pandas DataFrame is now an output for some methods.
I also set acc_type as demo as default because I think it's safer
a tests directory was created and there is a little nosetests sample.
`create_session()` must now be called outside of `__init__` (if we want to change
`parse_response` or `return_dataframe` before creating a session.

Kind regards

Femto