Increase text leading in Post Editor for better readability

Reported by a user comparing Google Keep and WPAndroid editors:

![3jt3rufofj-3000x3000](https://f.cloud.github.com/assets/40213/1550160/70b81d30-4dfc-11e3-9c52-d4e87da9c316.png)

![zmcwztdtl4-3000x3000](https://f.cloud.github.com/assets/40213/1550159/70b7d4e2-4dfc-11e3-9e22-924cd00fdf8b.png)
