Adjust width hueristic for Numeric Axis.

Now draws all ticks and takes the longest, rather than speculating
as to which will be the longest.

Added tests for width() and height() on Numeric Axis.

Added some documentation for Custom Formatter.

Close #604.
Add missing semicolons.
Merge branch 'master' into 604-numeric-axis-width
Merge branch 'master' into 604-numeric-axis-width
Switch to using TextMeasurer.
Merge branch 'master' into 604-numeric-axis-width
Adjust calculation to account for negative signs and variable-length formatters.
Merge branch 'master' into 604-numeric-axis-width

Conflicts:
	plottable.d.ts
	plottable.js
	src/utils/formatters/customFormatter.ts
Merge branch 'master' into 604-numeric-axis-width
Merge branch 'master' into 604-numeric-axis-width
Simplify calculation, add gutter(), remove old axisTests.ts

The calculation now just takes the max over all tick lengths.
While this will truncate some things if the user zooms in, it will
work correctly for static cases.

gutter() has been added to set the amount of extra space between
the tick labels and the edge of the axis. gutter() should now
be the preferred way to give extra room for decimal places on
zoom, rather than explicitly setting the width with width().

axisTests.ts was still hanging around, so I got rid of it.
Merge branch 'master' into 604-numeric-axis-width

Conflicts:
	plottable.d.ts
	plottable.js
	test/tests.js
Reword documentation, use clearer max() call, remove commented-out line.
Merge pull request #774 from palantir/604-numeric-axis-width

604 numeric axis width