Improve padding and alignment in the table

See #470 for a before screenshot. I made the Threshold (for Interval Classifications) and Value (for Value Classifications) columns center aligned, and added a padding of 2 whitespaces around the intervals in the Label column for Interval Classifications. After:

![table_after](https://f.cloud.github.com/assets/2470768/993207/ba5c708c-0994-11e3-8ea4-56c12c337b1f.png)