Features:
* Download attachments to cache folder (#181)
* Regularly delete attachments for deleted notifications (#142)
* Translations to different languages (#188, thanks to @StoyanDimitrov for initiating things)

Bugs:
* IllegalStateException: Failed to build unique file (#177, thanks to @Fallenbagel for reporting)
* SQLiteConstraintException: Crash during UP registration (#185)
* Refresh preferences screen after settings import (#183, thanks to @cmeis for reporting)
* Add priority strings to strings.xml to make it translatable (#192, thanks to @StoyanDimitrov)

Translations:
* English language improvements (thanks to @comradekingu)
* Bulgarian (thanks to @StoyanDimitrov)
* Chinese/Simplified (thanks to @poi and @PeterCxy)
* Dutch (incomplete, thanks to @diony)
* French (thanks to @Kusoneko and @mlcsthor)
* German (thanks to @cmeis)
* Italian (thanks to @theTranslator)
* Indonesian (thanks to @linerly)
* Norwegian (incomplete, thanks to @comradekingu)
* Portuguese/Brazil (thanks to @LW)
* Spanish (thanks to @rogeliodh)
* Turkish (thanks to @ersen)

Thanks:
* Many thanks to @cmeis, @Fallenbagel, @J117 and @rogeliodh for input on the new attachment logic, and for
  testing the release
