Updated the code based on Intellij 13.1

Checked the commit logs for https://github.com/JetBrains/intellij-community/blob/135/platform/lang-impl/src/com/intellij/find/actions/ShowUsagesAction.java file and updated the plugin. Seems to work as normal. 

I uploaded a new jar but kept the same version code.
By the way, the plugin is really nice!!

Signed-off-by: Carlos Sobrinho <carlos@sobrinho.pt>