Added missing dependency

Currently build would crash with bellow error due to annotation import not recognized
````
[ERROR] /Users/Peter/github/ShowcaseView/library/src/com/github/espiandev/showcaseview/ShowcaseView.java:[5,26] package android.annotation does not exist
[ERROR] /Users/Peter/github/ShowcaseView/library/src/com/github/espiandev/showcaseview/ShowcaseView.java:[490,10] cannot find symbol
symbol  : class SuppressLint
location: class com.github.espiandev.showcaseview.ShowcaseView
[INFO] 2 errors 
[INFO] -------------------------------------------------------------
[INFO] ------------------------------------------------------------------------
[INFO] Reactor Summary:
[INFO] 
[INFO] ShowcaseView (parent) ............................. SUCCESS [1.360s]
[INFO] ShowcaseView (library) ............................ FAILURE [3.704s]
[INFO] ShowcaseView (sample) ............................. SKIPPED
[INFO] ------------------------------------------------------------------------
[INFO] BUILD FAILURE
[INFO] ------------------------------------------------------------------------
[INFO] Total time: 6.554s
[INFO] Finished at: Tue May 07 17:05:03 BST 2013
[INFO] Final Memory: 15M/81M
````
Adding above dependency solves the issue