Use sed to replace /App/ with /src/

This covers most of the work that will need to be done to the docs for
the App -> src change in cakephp/app.
Fix App/ -> src/

This fixes another big gob of paths for the recent App->src directory
changes in cakephp/app
Fix a few more directory references.
Update based on review feedback.

@ADmad caught a pile of mistakes that I missed.
Fixing some extra src/ in paths
Fixing paths in installation instructions
Merge pull request #1439 from cakephp/app-src

Update App -> src