Updated build instruction link to the one on the Dev.Ardupilot wiki

Added detail for compiling with new version Arduino from version 3.1 /2.75

Updated the README and the install-prereqs-ubuntu.sh script to reflect
changes in the PX4 build steps and the PX$ & NuttX repository used to
build ardupilot.

pch: rebased pullreq #189

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

pch: rebased pullreq 183

  * Converted to Markdown.
  * Organization fix: Instructions on how to get the source come
    first.
  * Fixed old Google code URL to point to github instead.
  * Grammar fixes.
  * Added examples.