Add custom tooltips

Added the ability to specify a tooltipGenerator function in the config, to allow for customized tooltips

When a tooltip is meant to be displayed, this function will be passed an object with a `value` field, and (as a best effort) `index` and `series` fields. The function must return a string.

The modifications to `dist/angular-charts.js` have been tested in Firefox, Chrome, and Safari and seem to work fine. I don't have the compiler setup, and I can't run directly off the source file (see #15) so the modifications to `src/angular-charts.js` is a best guess.