Added support for ujson (if available) and added /pagedjson page

[jbtronics]

Added support for ujson (if available) and added /pagedjson page which seperates the json files into pages. (default 50 items per page). With the "items" param you can specify how many items should be shown on a single page. With "page" you specify which page number to show.

[koniu]

Hey, sorry for delay and thanks for the patch! I like the functionality (and haven't tried Recolldroid tho it sounds like a great initiative). On principle, I'm happy to merge but the PR has two problems:

1. the main patch introduces lots of code repetition, there's bound to be a better way
2. the changes to README.rst are unnecessary