Adds logging to the FakeServer

To enable (in node or a modern browser):
server.enableResponseLogging = true;
sinon.log = console.log;

This adds to the fake server the capability of logging responded
requests. This feature is opt in. What will be printed is a beautified
stringify of the request and response objects.

If the current environment does not support JSON.stringify the logging
fails silently.
Revers changes made by fccbac45ccb2c366f0eb1fec65f95e3ce8b37b2b

By some reason, in github, all the lines appear as changed but that is not the case and in
my machine only the lines I added apper in the diff.
Going to do every again to see if it works.
Adds logging to the FakeServer

To enable (in node or a modern browser):
server.enableResponseLogging = true;
sinon.log = console.log;

This adds to the fake server the capability of logging responded
requests. This feature is opt in. What will be printed is a beautified
stringify of the request and response objects.

If the current environment does not support JSON.stringify the logging
fails silently.
Now uses sinon.format to format the object instead of JSON.stringify

By using sinon.format it is no longer necessary to have logic that feature detects JSON.stringify
and the formatting is now consistent with the rest of the library.
Removes the usage of a flag to activate the fake server logging

Its necessary to opt in by setting sinon.log, so this flag was redundant.
Merge pull request #222 from feedzai/master

Adds logging to the FakeServer