Download automatically the BuildTools and support library

JakeWharton is the boss, once again :)

build.gradle

@@ -5,6 +5,7 @@ buildscript {
     }
     dependencies {
         classpath 'com.android.tools.build:gradle:1.0.0'
+        classpath 'com.jakewharton.sdkmanager:gradle-plugin:0.12.+'
     }
 }
 
@@ -12,9 +13,10 @@ allprojects {
     repositories {
         jcenter()
     }
+    apply plugin: 'android-sdk-manager'
 }
 
 ext {
     buildToolsVersion = "21.1.1"
     compileSdkVersion = 21
-}
+}