rename ChartController

I suggest renaming the internal controller to something more specific - e.g c3ChartController.

I was testing c3-angular for the first time and so I used the generic ChartController name without realising it was already being used by c3-angular. This caused a naming conflict and the whole app fell over with very little helpful errors (i.e. angular didn't say there was any namespace conflict, just that a bunch of functions weren't found). It wasn't until I started digging in the source file and API that I realised the name conflict was to blame.

Given that people are likely to use generic names like this in their apps, I would strongly suggest you use a much more explicit name for the internal controller - or make it much clearer in the documentation.