Compilation instructions

[gaul]

To compile Shortyz I needed to remove the androidkeyinfo.properties and signingConfigs parts of app/build.gradle. Could we modify these to make it easier for other contributors to work on Shortyz? This would enable other services like Travis continuous integration.

The README.md should also include any other setup such as:

echo sdk.dir=/path/to/android-sdk-linux/ > local.properties
JAVA_HOME=/path/to/jdk-7 ./gradlew clean assembleDebug