Rotate x axis labels

Added the ability to specify a rotation angle for x-axis labels, and to change the default margins (once the label is rotated, the margin probalby needs to be bigger).

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.