Styling suggestions

Hi

I would like to suggest a few style changes. This aren't exactly big issues, but I think they would improve usability a little bit.

* regular text field 100% width
* *Add row* button bigger or make use of *button-primary* class
* background color for group title, title field, to more easily distinguish new section/group
* if only one row present, don't show border-bottom on the row
* same top and bottom spacing/padding on field row
* ...

[As noted](https://github.com/WebDevStudios/CMB2/blob/master/CONTRIBUTING.md), I would like to ask if this is something I could start working on and of course submit a pull request?

A quick mock-up just for an example:
![repeater_mock](https://cloud.githubusercontent.com/assets/240953/4221029/cb96c5d6-3905-11e4-9ee0-507383e04c92.jpg)
![group_repeater_mock](https://cloud.githubusercontent.com/assets/240953/4221478/42d47086-390a-11e4-855d-cb732f2404bd.jpg)

Sincerely
Senicar