Variant symlink improvements

  * Add support for default values in variant symlinks
  * Add support for extended attribute "rawlink"
Change default catalog TTL to 15 minutes
Merge pull request #288 from reneme/fix_slc6_x64_test

FIX: Missing Permissions for `chcon` on /srv
opportunistically clean up before loading files > 25M
Merge branch 'master' of github.com:cvmfs/cvmfs
start development of cvmfs python library
add .cvmfspublished into _common
abstract Repository for addition of 'RemoteRepository'
add a manifest (.cvmfspublished) abstraction class
add remote repository support
adapt global collection functions
FIX: non-available RemoteRepository throws HTTPError