ENH: Add is_online property to easily determine whether we are online or not

[hhslepicka]

Closes #35

In [1]: from botcity.maestro import *

In [2]: maestro = BotMaestroSDK()

In [3]: maestro.is_online
Out[3]: False

In [4]: import os

In [5]: SERVER = os.getenv("BOTCITY_SERVER")

In [6]: LOGIN = os.getenv("BOTCITY_LOGIN")

In [7]: KEY = os.getenv("BOTCITY_KEY")

In [8]: maestro.login(server=SERVER, login=LOGIN, key=KEY)

In [9]: maestro.is_online
Out[9]: True

Attn. @morgannadev