Fixes #306: Can't set customlevels to my loggers

(aka `RangeError: Maximum call stack size exceeded` when setting custom levels)

I've tracked the issue and found that `log` method was overriden by one of the levels named `log`. My approach to fix this was creating a check and throwing an exception when a name collision is found. Also, I've added a `console.warn` when a level is overriding an logger object property (such as extend). 

@indexzero @jcrugzz we can discuss on Tuesday:

* [x] If this is the right fix or if we should fix it in another way.
* [x] I'm not 100% convinced that doing `console.warn` when methods are overriden by levels is the right choice. We have to discuss what to do in those cases.