Miscellaneous fixes

- Added checkout instructions for RecoLuminosity/LumiDB in addpkg_5_3_11.csh
- Added hltCandidateDiffChecker related stuff in hlt_MET_dev.py
- Fixed a typo in skimHLTL1ETM40_cfi.py
- Added skimHLTPFNoPUHT350_cfi.py