Fix code inspection results discussed in #67  - UrlShortener.java

Changes
-- Remove unused code
-- Fix unchecked call for Android AsyncTask execute() by specifying the generic types
-- Check for NPE

Please review changes and provide any feedback. 
your indent level is 4, the rest of the file is 2. Keep it at 2.

Prefrences->Code Style->JAVA indent -> 2, 4