Update styles to support using attributes for timeline elements

[walterg2]

Problem

Currently, the style sheet in place has a few places where it is looking for the timeline-* tags instead of relying on CSS classes. This causes users that want to keep their HTML valid issues as they need to duplicate the styles for angular-timeline.

Changes

• Updated SCSS file to support using attributes
  • Cleaned up the SCSS to take advantage of nested classes
  • Swapped the img style to be nested inside the timeline class so it would not collide with other styles set up by the user
• Added an example that renders the timeline using only HTML data attributes

Notes

• Resubmitting this based on the 1.7.0 release
• I did not include updated ngdocs due to it not correctly interpolating the links
• Package version was not bumped for this. While I believe this would fall under a revision classification, I felt it best to leave it up to you on how you wanted to update the version

Please let me know if you'd like any changes to what I've done here.

[walterg2]

Hey @rpocklin: Sorry for the long delay in getting back to this... kids and work got extremely busy over the summer.

I've made the fixes you asked for and updated everything to align with the 1.7.0 release. Let me know if you see anything else in here that needs tweaked.