Use domain "tools" for translations
Remove translations for development messages
Created `tools.pot`

* used `cake i18n extract`
Removed existing `.pot` files in favor of new `tools.pot`

* affected files: `captcha.pot`, `change_password.pot`, `confirmable.pot`
Merge existing single `.po` files to one `tools.po` each language

There were single `.po` files which contained only a few translations.
Now, they have been merged with `tools.pot` containing the strings which
have already been translated, but also all the other strings of the plugin.

I used `msgmerge --no-wrap $existing tools.pot`

* German: `captcha_de.po`, `change_password_de.po` and `qlogin_de.po` have
  been concatenated and merged with `tools.pot`
* Czech and Spanish: `change_password_cz.po`/`change_password_es.po` have
  been renamed and merged with `tools.pot`