Add __repr__ to Json object for better logging

[psycoteam]

Originally submitted by: Oleg Anashkin

Currently, attempt to log or print Json object uses default python's implementation which is useless. Using actual serialized content is much better.

I have sent a pull request on github with the proposed change: #9

[dvarrazzo]

As per discussion on github, Json.__str__ should return getquoted() as many adapters already do. Also note:

1. other adapters should be checked too (range?)
2. are we returning the right types in py3 (bytes/str)?

[dvarrazzo]

Pushed a str implementation for Json in my master. repr won't change.

I'm not planning to release the change in 2.5 as it may change the programs behaviour in a radical way: the idea is to release it in 2.6.