Use {}/format instead of % for numerical ngettext strings

[Drudenhaus]

Addresses #599.

[Drudenhaus]

I can reformat per the suggested changes in the travis output, but is make pot the correct way to update the line numbers in the translation files? It made a ton of line reference changes to the translation files that are unrelated to the changes I made. e.g.

sean@drudenhaus ~/workspace/exaile $ git diff --shortstat
 77 files changed, 8893 insertions(+), 10214 deletions(-)