localStorage mapped on android sharedpreferences

The goal of this PR is to add implementation for localStorage binding into Ejecta-X

setItem
getItem
removeItem
clear

are implemented

It uses the SharedPreferences storage of Android (the basic key value storage for mobiles)
(See: http://developer.android.com/reference/android/content/SharedPreferences.html)

Code to test it : 
````
  var score = 42;
  var bestScore = localStorage.getItem("my.bestScore") || 0;

  if (score > bestScore) {
    bestScore = score;
    localStorage.setItem("my.bestScore", score);
  }
````