add 'list-printers' and 'change-printer' commands.

the list-printers command shows a list of available printers as well
as the active printer in the list. printers are drawn with a name as
well as a short description.

change-printer can be used to change the printer by the name found in
the list-printers command.

notes:
- I'm not sure about the name `Pry::Printer` because we have `Pry::ColorPrinter` and traditionally the printer proc's have been called "print", not a "printer".

screenshots:
![screen shot 2014-03-21 at 07 18 25](https://f.cloud.github.com/assets/77573/2480550/10b72638-b0c1-11e3-843d-01794903403d.png)
![screen shot 2014-03-21 at 07 21 44](https://f.cloud.github.com/assets/77573/2480551/1b3432a4-b0c1-11e3-80a5-be368b21a707.png)