Fix download of certain add-ons, plus documentation improvements

* Added --all to the sdk command to ensure that all packages can be installed, despite their obsolete/dependent status.
* The doc claims that OSX version installs in /Developer/android, however (at least in the most recent version) it installs in /usr/local/android on OSX too.
* Added a specific add-on example that helps define the add-on names to use.
* Also added a tip to get the names of the add-ons in a list.