Import the LruCache class instead of the whole support lib.

The v4 support library was only use for the LruCache class. Instead of
importing the whole library in the DataDroid library project, the class
is now copied in the project.

It's going to remove the conflict if a newer support lib version is
released
Improve some documentation

- In MultiThreadedIntentService, the method getMaximumNumberOfThreads now
clearly states that it is called internally only once during onCreate
- In RequestManager, a link to the datadroidlib.com website is added to
allow the developer to find a tutorial on how to use it directly from
inside the source code
Fix the proguard file of DataDroid project
Fix one URL and update version to 5/2.1