1142824: Fixed Java gettext extract task

Added the -k option to the xgettext command in gettext.rake. This
tells xgettext to ignore its default methods for export. Specifically,
this should cause it to no longer extract configuration keys used
with the getString method of our AbstractConfiguration class.
Modified paths displayed in gettext output.

- The gettext extract task now only lists source files relative to
  the project root, rather than the absolute path.
- Added the -F flag to the xgettext command to ensure keys are
  sorted by the file (and line) in which they originate.
Merge pull request #730 from candlepin/crog/1142824

1142824: Fixed gettext:extract buildr task