Add CHANGELOG.md and /res/values/changelog.xml

* fix for #1161
* historical data retrieved from https://web.archive.org/web/*/https://play.google.com/store/apps/details?id=org.thoughtcrime.securesms
* added xml version to enable translation in transifex (as proposed by @generalmanager in https://github.com/WhisperSystems/TextSecure/issues/1295#issuecomment-39633459 )