installation: Python-2.6 and pip requirements

* Increases minimal Python version requirement to v2.6.  There will
  be soon lots of code merged to the master branch, coming from INSPIRE,
  that is no longer Python-2.4 compatible.  The time has come
  to abandon Python-2.4 compatibility!

* Inclusion of pip requirements files suitable for the master branch.

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