installation: README.rst with build status badges

* Adds new documentation file README.rst which is essentially
  identical to README, plus contains extra links to Travis build
  status badges, bringing them onto the GitHub home page.

  This is done because apparently GitHub only recognises file
  formats by extensions, so badges cannot be easily introduced
  into plain README files (that we still keep as per GNU autotools
  practices).

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>
installation: GnuPG key server location update

* Updates location from where to get the GnuPG keys; the old
  location was not working anymore since quite a while.

Signed-off-by: Tibor Simko <tibor.simko@cern.ch>