Make habit +/- buttons fill height and vertically centered

Plus/minus buttons had a fixed height, but that was inconsistent with all other task types. This patch makes the buttons fill the full height, as well as centers the labels vertically.

Discussion of this change in pull request #3509

## Before
![screenshot 2014-05-21 14 44 51](https://cloud.githubusercontent.com/assets/44676/3047129/cb1ee2d6-e131-11e3-8414-606cd08f8fdd.png)

## After
![screenshot 2014-05-21 18 35 30](https://cloud.githubusercontent.com/assets/44676/3048900/64e3fa40-e151-11e3-8187-f2a9f6352665.png)