Habit +/- buttons fill height like other tasks

Plus/minus buttons had a fixed height, but that was inconsistent with
all other task types

## 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 14 46 02](https://cloud.githubusercontent.com/assets/44676/3047132/cf022598-e131-11e3-9465-0c4f7dca063f.png)