Changed Base::callConstructDocument sequence parameter to std::size_t
* "unsigned" type led to a compiler error on 64bit systems
** the Sequence template uses std::size_t which resolves to "unsigned long" instead of "unsigned int" on 64bit systems
** as "unsigned" is resolved to "unsigned int" by the compiler this situation produced a type conflict
* added recommendation for using a xalan library version built with ICU flags to README.md
** otherwise e.g. the "format-number" function is not fully supported