Add env-read-docs event to allow modify docnames list before processing
Add unit test for new env-read-docs event
Don't multiply docnames list size per each listener of env-read-docs

The env-read-docs listener function doesn't return the modified docnames list
but change the list received as parameter.
Updated the tests
Merged in guillemNaN/sphinx (pull request #261)

Add env-read-docs event to allow modify docnames list before processing