Added context awareness to XObjectValue casting logic
* added support for defining boost::filesystem::path as a external function parameter
** boost::filesystem::path parameters are resolved against the appropriate FilesystemContext and IncludeEntityResolver instances
* xalan::XSLTInputSource parameter source paths are also resolved
* removed need for passing a reference FilesystemContext to "constructDocument" methods
** they now only accept the parameters of the external function implemented by them
** all path resolution logic is wrapped by the newly created XObjectValue class
* converted XObjectValue namespace into class
** the "get" template method is now a template member method
** this was needed to enable value casting logic to access the appropriate FilesystemContext and IncludeEntityResolver instances
* this commit marks the next step towards the goals defined in 741a70f