Update console.log to work with circula reference object

[nqbao]

I found a small bug when mocha-phantomjs reports test as failed while passed in browser. It turns out that i was using console.log to print out a circular object, and mocha-phantomjs console.log function fails to handle that. The bug can be illustrated as follow

var o = {};
o['self'] = o;

console.log(o);

This pull request fixed the bug and also add a unit test to cover that bug.

I hope this will be merged soon!

[nathanboktae]

Nice submission, thanks!