Add in NumberAxis.

Axis for showing numerical values. Tries to calculate a width/height
on layout, but user can override using width()/height() (on BaseAxis).

Currently missing tick-positioning functionality from the original Axis.
Tests coming shortly.
Move width/height-setting and computedWidth/computedHeight to base axis

Now users can call width("auto") and height("auto") to restore automatic mode.
Fix inversion error in height() call.
Add in tests for NumberAxis.

Also add in option to hide end tick labels.
Actually include NumberAxis test file.
Tick labels can be repositioned.

Changed BaseAxis to draw tick marks in one <g> and tick labels in another.
  - This allows the tick labels to be repositioned as a group,
    and opens up the possibility of having multiple tiers of
    tick labels in the future.

Factored out _computeWidth() and _computeHeight()
  - These calls calculate the width/height of the axis, so
    the layout logic can live in BaseAxis.

Tick labels can be positioned relative to the marks on NumberAxis.
  - Functionality from the old Axis class, valid positions are
    [left/center/right] for horizontal axes and [top/center/bottom]
    for vertical ones.

End tick labels are hidden by default.

Added more tests.
Hide overlapping tick labels; width()/height() return actual width/height of axis

Imported hideOverlappingTickLabels() from the old Axis class. Going forward,
I'd like to address the problem of overlapping tick labels via more flexible
tick generation on the scales, as I think that would produce better results.

.width() and .height() now return the actually-used width/height if invoked
with no arguments.
Rename Axis.Number to Axis.Numeric.

Previously, the .d.ts file would indicate that a method that returns
the axis itself returns a "Number", which was super confusing. It now
returns a "Numeric" (somewhat clearer).
Merge branch 'master' into 517-number-axis

Conflicts:
	plottable.js
	src/components/baseAxis.ts
Merge pull request #564 from palantir/517-number-axis

Add in NumberAxis