Fixed issue #76 that throws an error when displaying charts in angular-u...

Hi Chinmay,

I fixed the issue #76, so that we can use angular-charts in bootstrap-ui modals. I also updated the readme.md to show how to use the new config key and extended the unit tests to cover the new case.

As it might take a while to reproduce the issue #76, and to check how this PR request works, I created 2 Plunkers:
- Before the fix: http://plnkr.co/edit/YlWuJN?p=preview
- After the fix: http://plnkr.co/edit/84zuEg?p=preview

Please let me know if I forgot to update something.