Now all installers use a temporary directory

xdrfile source has been removed from gromacs ftp

I put a copy on people.sissa.it/~bussi/plumed so that we
can download it on travis

there are too many travis files. I moved all of them
on a .travis directory (except for .travis.yml that should
be in root directory)

Travis test implemented base on a branch originally made by @tonigi

Every push on a branch containing this file will trigger a build
on travis CI. Several test configurations are attempted
(with/without MPI) and regtest are run. Also manual is generated.