DONE: Graph service

Problem
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Graph needs clean up and the client code contains lots of duplicate d3 code

Solution
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Create a graph service to draw graphs. And create a higher level nxt-d3-service to create recurrent tasks such a creating axis and scales.

Actions
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- [x] Create nxt-d3-service
- [x] Migrate code from timeline service to nxt-d3
- [x] Adapt timeline service to use nxt-d3
- [x] Create graph-service
- [x] Rewrite code from directives to graph-service using the nxt-d3-service
- [x] Rewrite graph directives to use graph-service and clean up $scope.watches
- [x] Fix intersect
- [x] Fix css
- [x] Write docs
- [x] Write tests