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:

* [ ] If this is the right fix or if we should fix it in another way.
* [ ] 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.
Don't really need this. The user is explicitly saying overwrite these levels so they _should_ know what they're doing.