Added spanish as new lenguage

More information here:
https://www.transifex.com/projects/p/app-android/
Merge pull request #413 from alexsalas/master

Added spanish as new lenguage
Various bugfixes and improvements.
Cleanup readers.xml in Spanish.
Fix #395 - next/previous buttons in translations.
Swap previous and next buttons in translations.
Bump vesion to 2.5.7, update strings.
Fix tablet landscape being broken.
Bumping version to 2.5.8.
Fix bug with long press. Cleanup some code.