How to Use Arduino to Create a Programmable Keyboard?

I want to connect a USB keyboard to my Arduino and then connect my Arduino to my computer via USB so that my Arduino is recognized as a keyboard. This will allow me to remap the keys.

Required Hardware for This Setup:
[Keyboard] -> [Arduino (As Virtual Keyboard)] -> [Computer]

The goal is to achieve this configuration as cheaply as possible. 😀

• Arduino Pro Microthat use the chip atmega32u4 and that can do comunication by USB as a virtual keyboard
  
• USB HOST board like this to get ANOTHER usb connection in wich I can connect the real keyboard.