PETSc 3.5.1 with intel 2014b

Plus dependencies
Needs https://github.com/hpcugent/easybuild-easyblocks/pull/446
and https://github.com/hpcugent/easybuild-framework/pull/987


We already have one apparently: ```LAUNCHPAD_SOURCE``` is ```'https://launchpad.net/%(namelower)s/%(version_major_minor)s.x/%(version)s/+download/```

A quick grep learns that there are 3 or 4 different download urls for launchpad... 
@boegel Can we give a tuple to a template or a list of alternatives?