Fix template plural detection
Fix detection of removed files for translation updates

Translation reloading was not affected because the list of files to watch
sent to the FileWatcher was generated and not listed from existing files.
Fix tests not executed because of wrong function name

Error caught using coverage.py
Add test for obsolete pruning
Add context and plurals in integration test
Avoid modifying dictionary while iterating on its keys

It's ok on python 2 but not on python 3 where keys() returns
an iterator instead of a list.