Let travis build signed release-APK's

Created by: larsgrefer

Steps:

• Adjust the app/build.gradle-file to read the sensitive keystore passwords from properties
• Add the (encrypted) keystore to the repo
• Set special environment variables on travis in order to set the above mentioned gradle-properties

Possible Benefits:

• Travis can automaticly attatch the APK to github releases
  • https://docs.travis-ci.com/user/deployment/releases
• Travis can deploy the apk everywhere we want:
  • https://docs.travis-ci.com/user/deployment/custom/
  • https://docs.travis-ci.com/user/deployment/script/