Add Slovak translation
Validate values returned by K-9 Mail's content provider

 This should fix a NPE reported via Google Play.
Update README

* Update changlog
* Add list of contributors
v1.2
Add italian translation
Update contributors list in README
Merge branch 'translation_it' into dev
Open Play Store app when extension is clicked and K-9 Mail is not installed
Remove useless object creation
Remove code that is unnecessary because of minSdkVersion 17
Show error messages in settings screen
Display error message when we have no permission to access K-9 Mail

When the extension is installed before K-9 Mail it won't have the needed permissions to
access K-9 Mail's content provider. Reinstalling or updating the extension fixes this
problem.
Update German translation
Don't hide accounts without a name
Update changelog in README
v1.3