ixyml 0.0.1

I propose a new file format called "XYML." Xyml module has the following functions: * loads a XYML file as an instance of Xyml::Document, saves an instance of Xyml::Document as a XYML file. * loads an XML subset file as an instance of Xyml::Document, saves an instance of Xyml::Document as an XML file. * saves an instance of Xyml::Document as a JSON file. * converts an instance of Xyml::Document to an instance of REXML::Document and vice versa. The instance of REXML::Document supports a subset of XML specifications. Xyml_element module provides XYML element API methods. These API methods can be used to elements in Xyml::Document.