Remove Logger from core module

[takahi-i]

For the handy use of document-validator as a library, we remove logger from core module. The errors in core are going to be thrown and caught by the applications.

[jmoriya]

I think Logger.error can be replaced as some kind of exception (DocumentValidatorException or standard exception).

And, Logger.warn and Logger.info needs somewhere to need to save if we have to keep their message.
My personal one idea is that DocumentValidator itself provides warn and info.
Does it make sense? Or is it too much?