implement addpkg -f FILE functionality

This is on top of #37 and #38 .

Implement in git-cms-addpkg the functionality of "addpkg -f FILE" .

Note that FILE can contain any number of
 - comments (lines starting with #)
 - empty line (lines with only white spaces)
 - subsystems (e.g. FWCore)
 - packages (e.g. FWCore/Version)

Leading and trailing slashes are stripped, so these are valid, too:
 - //FWCore/
 - FWCore/Version///
 - etc.