fix axis postitioning bug (issue #420)

When an axis position was changed to "top" or "bottom", the axis gets
longer. But when it gets changed back to "middle", it stays long.  The
same applies to horizontal axes.

This diff makes it get shorter again.
Merge branch 'master' into axis-position
Axis._DEFAULT_TICK_SIZE = 6
Merge branch 'master' into axis-position
Update built files
Update built files
Merge branch 'master' into axis-position
Merge branch 'axis-position' of https://github.com/palantir/plottable into axis-position
Merge branch 'master' into axis-position

Conflicts:
	plottable.d.ts
Merge pull request #451 from palantir/axis-position

Axis position issue #420