Better differentiate Humhub from Google Calendar

[frommie]

In humhub.py there are several mixings of getting calendar items from Google and processing them. Better way would be to differentiate more using a config file to in future you could not only access google calendar items but other calendar services (like from humhub itselfe).
Therefore the respective methods and data types should be abstracted and standardized.

[frommie]

Example: https://github.com/frommie/rasahub-humhub/blob/master/rasahub_humhub/humhub.py#L150 - this should be abstracted from calendar service