Modified startTag regex to allow for numbers in namespace prefixes.

According to the XML W3C spec recommendation (http://www.w3.org/TR/xml-names11/#NT-Prefix) a namespace prefix can contain digits, but the ```startTag``` regex doesn't currently allow for this. Without this change, jsxml throws ```[XML Parse Error] the start tag is invalid ...``` for an XML tag such as ```<ns3:someTagName> ...```