Improved log() function to handle "undefined" as well as object's of type "xml"

Currently (1.0.1 and below), if you try log(undefined) in a map/reduce or other JS function in CouchDB, you'll get a rather verbose stack trace. This patch casts the message var as a String to avoid that stack trace and output the text representation of "undefined".

Additionally, this patch includes outputting XML objects (build with E4X: new XML('<test />')) as string representations of the entire XML (using toXMLString()).