Support timeout arguments for all method using requests

[Jasonnor]

Issue description

Without a timeout, our code may hang for minutes or more.

(For example, the owncloud URL is set incorrectly or the service is unavailable)

Possible solutions

Set a timeout argument in Client class and pass it in all method using requests.

class Client(object):
    def __init__(self, url, **kwargs):
        ...
        self._timeout = kwargs.get('timeout', None)

    def _make_ocs_request(self, method, service, action, **kwargs):
        ...
        res = self._session.request(method, self.url + path, timeout=self._timeout, **attributes)
        return res

If you don’t mind, I can open a pull request to solve it.