title: Android Studio: underscores in literals are not supported error
body: <p>I have a problem with the drawable class in  R.java file. I had drawable hdpi, mdpi, xhdpi... folders in main/res, but drawable was missing so i created it by hand. Main.xml is able to find the image declared but this code in R.java gives an error:  </p>
  
  <pre><code>    public static final class drawable {
      public static final int 10_device_access_accounts=0x7f020000;
      public static final int 10_device_access_add_alarm=0x7f020001;
      public static final int 10_device_access_alarms=0x7f020002;
      public static final int 10_device_access_battery=0x7f020003;
      public static final int 10_device_access_bightness_low=0x7f020004;
      public static final int 10_device_access_bluetooth=0x7f020005;
      public static final int 10_device_access_bluetooth_connected=0x7f020006;
      public static final int 10_device_access_bluetooth_searching=0x7f020007;
      public static final int 10_device_access_brightness_auto=0x7f020008;
      ...}
  </code></pre>
  
  <p>When compiled gives out an error: 
  Gradle: error: underscores in literals are not supported in -source 1.6
  (use -source 7 or higher to enable underscores in literals)</p>
  
  <p>I'm using android studio version 1.0.0.1 and jdk 1.7.0.250 version, i have PATH variable pointing to %JAVA_HOME%\bin and CLASSPATH pointing to %JAVA_HOME%\lib</p>