Fix: Conditional formatting because gauges can also contain strings

Fixed issue that values of gauges with non-number values were not
displayed in the table.

* added textOrNumber filter in app
* For Gauges replaced {{value.value | number:0}} with {{value.value |
textOrNumber:1}} in overview.html
small refactorings in GraphDataGenerator

* removed catch for unthrown exception
* added generic type information
* add log statement in case of error when graph is build
Merge pull request #4 from fexbraun/master

Allow string values in Gauges