Added function Parser::LoadFile.

It is possible to load a file with the following structure:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<project name="myproject">
<src path="file_cpp_test" /><!-- ... -->
<hdr path="file_h_test" /><!-- ... -->
<incdir path="include_test" /><!-- ... -->
<libdir path="lib_test" /><!-- ... -->
</project>