Add support for data series with gaps

This request allows to force the frontend to stop interpolating when data points are missing. A new optional property `gap` indicates the minimum number of seconds the frontend should consider when displaying a time series as interrupted. 
Useful for data that is e.g. only available during operating hours (like solar systems power or voltage). Not available for entities with `hasConsumption=true`.

Example:

![unbenannt](https://cloud.githubusercontent.com/assets/184815/3095255/85a102e6-e5c5-11e3-880a-2e91988ba51f.png)