Vertical centering and padding

I made the text center when the icon is bigger than the text:
![centered text](https://cloud.githubusercontent.com/assets/5983710/3125246/4c20aad0-e787-11e3-914a-ae25fbf9ea1d.png)
This should solve issue #152.

I also edited the way the frames are drawn and the padding is made, with the current behavior icons are not padded correctly when they are bigger than the text (they appear too low and can even overflow out of the window if the padding is low).
With those edits, the icon will appear centered if it's bigger than the text, as shown in the picture above.