Refactor time series charts API

Remove stacked bar options from time series charts, add left bar offset,
and perform general refactoring.

Author: Chris Bisignani
Reviewer: Dylan Nugent

Part of #40
Finish refactoring time series charts.

Finishes the work started by Chris on time series charts. Mostly just a
couple of bugfixes within that. Also remove documentation and example
options pointing to stacked bars as an API option for time series
charts, as they are removed by Chris's time series refactoring project.

Author: Dylan Nugent
Reviewer: Alex Zirbel

Part of #40
Fix and implement data offset options for time series bar charts

Previously, barLeftOffset didn't behave entirely clearly. Treat inputs
to barLeftOffset as data offsetting instead of just rendering offsetting
in order to properly draw bar charts where data points shouldn't be
drawn as the center of the bar.

Author: Dylan Nugent
Reviewer: Alex Zirbel

Part of #40
Implement more useful value labeling.

Add independent value formatting for axis ticks and hover values on
charts that have axes displaying values. Ticks display unrounded but
SI-prefixed values, ensuring that two ticks showing the same value due
to rounding to a significant figure below the tick delta never happens.
Hover values show value to fixed precision of 2 numbers after decimal,
and formatting value for hover only can now be overridden with
"formatLabel" instead of "formatValue" variable to reflect that this no
longer updates the value formatting used on the axes.

Author: Dylan Nugent
Reviewer: Alex Zirbel

Part of #40
Fix bug `Assertion failed: Expected hash or Mixin instance, got [object String]`

Author: Cyril Fluck
Reviewer: Dylan Nugent

Closes #41
Allow axis labels on time series charts to be centered

Add a setting "centerAxis" for the time series labeller which will place
time labels at the center of an interval instead of the start of that
interval. For instance, with "centerAxis" true, the label for March 2004
would appear at the tick on March 15 instead of Match 1.

Author: Dylan Nugent
Reviewer: Cyril Fluck

Part of #40
Implement sort-by-value on vertical bar charts

Allow sorting vertical bar charts by value order, similar to horizontal
bar charts. Will sort by the value of the first grouping in grouped
vertical bar charts.

Author: Dylan Nugent
Reviewer: Cyril Fluck

Part of #40
Fix code style in chart functions for clarity

Change hasBarData and hasLineData to use Ember.computed.notEmpty instead
of being explicitly written out functions, and rename variables that
return functions to "varFn" instead of "getVar" which was unclear. Code
cleanliness commit.

Author: Dylan Nugent
Reviewer: Cyril Fluck

Closes #40