Removed padding-top for rs-btn-link objects, except when in rs-btn-group

Previously, there was a padding-top of 2 pixels for all rs-btn-link elements. This caused them to be mis-aligned next to other text and links. The padding was intended to align rs-btn-link objects with other buttons, as in an rs-btn-group. I've removed the 2 pixel padding for rs-btn-link elements in general and added it back in for rs-btn-link elements within an rs-btn-group.

Text alignment before:
![link-alignment-before](https://cloud.githubusercontent.com/assets/805335/5093041/4483d4b4-6f22-11e4-9eeb-cf7be19e9bbe.png)

Text alignment after:
![link-alignment-after](https://cloud.githubusercontent.com/assets/805335/5093046/4ae1868a-6f22-11e4-9fcf-962923699d86.png)

Text alignment after (in a button group):
![form-alignment](https://cloud.githubusercontent.com/assets/805335/5093050/55eab268-6f22-11e4-9487-263cd6ec5eba.png)