Switch to Java 7.

Notes regarding Android compatibility:
- The 'try-with-resources' language feature needs API level 19, so don't use it.
  All other language features can be used back to API level 8.
- The JDK 1.7 API still isn't fully implemented, so better stick with 1.6 for now.

service/build.gradle

@@ -8,6 +8,6 @@ dependencies {
 }
 
 compileJava {
-    sourceCompatibility '1.6'
-    targetCompatibility '1.6'
+    sourceCompatibility '1.7'
+    targetCompatibility '1.7'
 }