More functions, improved protocol, cleaned code

Hi Ben,
I'm a large fan of your Open Device Manager. I've done several improvements to your project that you can hopefully use :) 

Changes in the Android App:
- Added PIN lock for the Settingsactivity
- Added AudioCapture functionality
- Users can request device metadata
- Users can send custom SMS texts
- All strings used in the app are extracted to a own strings.xml file -> multilanguage is possible
- changed versionchecking to look for new version on the odm webapp instead of github
- improved android - server protocol
- many other minor improvements