Parsing XSD files with imports of other XSD files seem not to work when parsing from memory

[wouterz]

The only function to parse a schema document seems to use a memory parser, which does does not seem to work well with external xsd files (especially if they are not in the document root).

I suggest to add additional functions to use a document pointer, which seems to support this functionality. (unless I have missed some way to do it with the current lib)

[wouterz]

opened a pull request which would add this: https://github.com/lestrrat/go-libxml2/pull/49