New visualization features

* [x] when zoomed out, only show root ops in fragment view
* [x] don't fetch data below pixel width
* [x] ~~limit number of ops returned in fragment visualization~~ not necessary yet
* [x] remove line chart from network view because nobody really used it
* [x] add visualization of how much time was spent in each operator
- [x] deal with long labels
- [x] ~~change arrow width according to amount of data sent~~ too complicated for now
- [x] ~~log area in treemap???~~ removed treemap
- [x] brushing on fragment overview
- [x] drill down into ops in overview
- [x] use js graph for physical plan in editor as requested by @billhowe 
- [x] subtract contributions from children from operators (phantom "really long apply" that actually does nothing)
- [x] strange zoom in second level of child fragment view.

See https://github.com/uwescience/myria/pull/536

Also #41