Part of speech pagination

Fixes #22

This code is ugly and can probably be improved a lot. Some points worth looking
at are:
    - It's using an html element as a kind of lock to prevent loading the same
      page multiple times (I'm so, so sorry for this).
    - The "json" format is being treated in a special way in our POS view, but
      what is really special is the paginatioJ. This indicates that maybe we
      need a view to build the structure of the POS tab, and another one to
      actually serve the pages.
    - The `part-of-speech.json' template is actually just rendering the dumped
      json. This is for the same reason as the point above.

@turicas *please* review this code after you've dealt with #58. After that pull request is merged, this should apply fine to develop.