Task/395 pagination queries

### Description
Pagination implemented and with rudimentary unit tests.
URI params 'offset' and 'limit' used.
A third URI param 'details' steers whether 'mongo db count' command is issued on no hits.

When you add a new commit, the files tab changes and some links may be lost if you have modified arround the diff where that comment what posted... However, the same comment remaisn always in the conversation tab: https://github.com/telefonicaid/fiware-orion/pull/414#discussion_r13659735

However, you are right, pobably plain copy&paste is better.