Update gradle build files to 0.7.3 android plugin, add Gradle 1.9 wrapper, add information about build to readme

build.gradle

@@ -4,7 +4,7 @@ buildscript {
     }
 
     dependencies {
-        classpath 'com.android.tools.build:gradle:0.6.3'
+        classpath 'com.android.tools.build:gradle:0.7.3'
     }
 }
 
@@ -15,5 +15,5 @@ allprojects {
 }
 
 task wrapper(type: Wrapper) {
-    gradleVersion = '1.8'
+    gradleVersion = '1.9'
 }