Add a tooltip component to perseus

Summary:
A semi-generic tooltip component. Allows placing a tooltip at any
of the four corners of a target, aligned in either direction.
Eventually should support centers of things too.

This should eventually be in react-components.

Screenshots:
{F16846}

{F16848}

{F16850}

Test Plan:
Made an InputWithExamples for the tooltip formats change,
and tried all eight position/alignment options

Reviewers: alex

Reviewed By: alex

Differential Revision: http://phabricator.khanacademy.org/D8757