Update plugin installation section
Refactoring RestTask

The server response now is saved inside an instance field, so that
child tasks can use it if needed
Now the validation of the input is done inside a dedicated method
Adds an additional check to assure that if you do a GET request you don't
set a request body
Fix the unit tests

Now the unit tests don't check response.getData(), since it's not
called anymore
Fixes the authentication header test, now it really checks the header
Merge branch 'refactoring' of https://github.com/SirBif/rest-gradle-plugin into refactoring