Fix "show collections" alignment and padding

Hey @TylerBrock,

Two minor changes to ensure that the alignment and padding used for the `show collections` command is correct, and corresponds to that used for the `show dbs` command.

It wasn't working for long collection names... see the attached screenshot, which shows the before and after behaviour.

![before_after_fix](https://f.cloud.github.com/assets/17322/1795596/8746a988-6a24-11e3-941c-a53d62188991.png)