Move www/ and merges/ into app/.

The structure is now:
app/
  merges/
    android/
    ios/
  www/
  config.xml

All code, tests and docs are updated and everything is still working. This
is a breaking change for old projects, but it's fairly easy to perform the
move manually:
$ mkdir app
$ mv www/config.xml app
$ mv www app
$ mv merges app