Added Vagrantfile to define a standardized Ubuntu 12.04 32-bit
development environment.

Added a script, install-prereqs-ubuntu.sh, that installs all packages
and software required for development on AVR and PX4 platforms.

pch: rebased pullreq 183
Updated README:

 * Added instructions on installing prerequisites in (Ubuntu) Linux.
 * Added instructions on how to use vagrant for development.
 * Minor copy-editing.

pch: rebased pullreq 183