Add exception declarations to methods on Xstream class

com.thoughtworks.xstream.Xstream does not declare any exceptions on its method declarations, either as throws clauses or as javadoc, even for methods such as fromXML(Reader), which should have a very clearly defined exception behaviour. As it stands, we're reduced to try/catch RuntimeException, which isn't very precise.

The javadoc should declare the various runtime exceptions that may be thrown.