Parse XML correctly

[skreutzer]

Make sure that the XML input is properly parsed. The current implementation is not guaranteed to handle the input correctly, as there are specific XML parsing rules defined, which aren't necessarily reflected in the code for now. There could also be an alternative mode which doesn't follow the official, standardized parsing rules, but complains about all < and > which aren't part of tags even where they would otherwise be allowed by the standard. Furthermore, parsing names could be an extra method in order to reflect rules like tag names aren't allowed to be empty, start with a number, etc. It would be a good idea to look at BNF grammar for XML.