Add plugin icons

Closes #121
Changelog for 1.5.1

(cherry picked from commit 15aa04230aa27d5790f2b4bee127f2b60768c47c)
Update WP L10n Validator config and caches
Add change log for 1.6.0
Tested up to 4.0
Version 1.6.0
Update POT file for 1.6.0