Correct the update of a git package.

This modification is useful in a tricky case:
When one is working on a branch whose corresponding
remote branch has been deleted, the pull will not work
Hence, I think it is better to fetch first (this step
may not be mandatory) then update.
Merge pull request #5 from francois-keith/master

Correct the update of a git package.
Add installation of doxygen external dependencies.
Correct the installation of doxygen/git.

The version checked should be the one given
in the parameters above, otherwise, this will
loop.
Correct ros installation and use of ros variables.

If ROS is not installed, it will be installed by
./install ros 2
The absence of ROS will not stop the script for
the 4 first steps of the installation.

Also remove useless ros variables. The redefinition
of ROS_ROOT was wrong for groovy and fuerte anyways.