Convert to the gradle build system

Ports the Maven build POMs to Gradle build scripts, which are a bit
friendlier to use.  Instead of using the exec:java Maven goal to run the
server now, the gradle run task should be used.

cache/build.gradle

@@ -0,0 +1,5 @@
+description = 'Apollo Cache'
+
+dependencies {
+    compile project(':util')
+}