Dylan/charts bugs

This includes @bigsley's refactoring work, my fixes to that, a centering option for bar charts so they can be rendered with end or start of period data as well as middle of period data, and a fix for rounding errors on labels.

@azirbel @bigsley 
Looking deeper into this I think the best solution is to offer `barLeftOffset` and `xAxisCentering` and remove `barCentering` (while refactoring `barLeftOffset` so that it correctly works to meet the needs served by `barCentering`). Basically the use cases the three things were trying to handle are:

* If bar data represents the start or end of a time period, we should be able to shift where the data is rendered on the axis. Since shifting the bars and shifting the data is cosmetically the same (and ember-charts does not and should not support rendering invalid charts), this case should be handled by `barLeftOffset`.
* Axis labels (ticks) should also be able to represent points within a time period that are desirable given the data; for instance if I have bar charts representing quarters I might want to draw ticks in the middle of a quarter or at the end of a quarter instead of at the start.

Seems to me like `barLeftOffset` and `axisCentering` (which I'm working on now) would be comprehensive solutions and `barCentering` is unneeded (or `barLeftOffset` should behave more like `barCentering`). @bigsley did you have a different use case in mind here?