Implement postLS1 customisation as an Era

This request implements SLHCUpgradeSimulations/Configuration/postLS1Customs.customisePostLS1 as an "era", aka cms.Modifier.  It doesn't change any of the existing configurations, just implements a duplicate way of applying the customisations.  Once this is properly tested it is expected that the customisePostLS1 customisation will be taken out, and this way of doing things extended to replace all of the other customisations.

To activate use the cmsDriver option "--era run2" instead of "--customise SLHCUpgradeSimulations/Configuration/postLS1Customs.customisePostLS1".

I've done some comparisons for the 1320.0 workflow, python dumps for both methods are in ~grimes/public/eraComparisonCheck.  There are some differences if you use a simple "dumb" diff, but if you follow the commands through the two methods are effectively the same, as of all customisation updates in the 2014-12-03-0200 IB.

As mentioned this just adds, and doesn't take out the old way of customising.  Once this pull request has been discussed the standard Run 2 workflows can be changed to use the era instead of the customisation.

@Dr15Jones @davidlange6