Various changes and bugfixes:

- merged schema.js and collections.js, otherwise the Schema object
  collections.js refer to is undefined. I could have managed keeping
  the files separate and modifying the schema.js file name to make it
  load before collections.js, but I preferred this way which seems
  cleaner. Moreover collections.js was just three lines anyway, no
  point in keeping it standalone.

- changed some types in the schema.

- fixed the addCommentToPost method