INFRA-104: create-pot-files-extensions.sh: add POTDIR env var.

Allow to set the potdir using the POTDIR environment variable.
This makes it easier to pass a specific script argument, but not need
to pass the other arguments which can be auto-detected by this script
using the info.xml of the extension.
header: bump copyright to 2014.
Merge branch 'master' of https://github.com/civicrm/l10n