support for numeric attributes/text & suppression of false-y attributes

added support for using numeric values in attributes and element text
added support for suppressing attribute if the value is false-y

xw.startElement('tag')
.writeAttribute('key1', false)
.writeAttribute('key2', null)
.writeAttribute('key3', undefined)
.text(3.14)
.endElement();

results in: <tag>3.14</tag>

(see https://github.com/touv/node-xml-writer/issues/5)