Fixing IO.binwrite specs, parsing options.

@hedius @enebo @atambo Fixing binwrites specs. And as the hasOffset flag was removed and the mode calculation was done inline on the binwrite method I felt the necessity to parse mode in a string format to an int oflag. I've found a deprecated method that does this conversion, I know it is not the perfect world if you can give me feedback on  other method that does the same thing. 

EncodingUtils has a method that kind of does the same thing, but it depends on a file instance which I don't have.

Thoughts?