autotest: bumped version number of QUpgrade

rebased (pch) - closes pull request 184
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
autotest: APM xml docs should be named xml.pdef.xml

(not arduplane.pdef.xml)

pch: rebased pull request #188
documentation: explain what a 'sketch' is.
documentation: explain where the build output goes.

pch: rebased pullreq #189
InertialSensor: added OilPan INS example
autotest: fixed sleep_overhead calculation in sim_multicopter.py