Fix java.lang.NoSuchMethodError

https://developer.android.com/reference/java/lang/String.html#isEmpty() is available only for Api 11 and not Api 9. 

Better use https://developer.android.com/reference/android/text/TextUtils.html#isEmpty(java.lang.CharSequence)
Merge pull request #19 from bunjix/patch-1

Fix java.lang.NoSuchMethodError