add an options argument to parseXml so that xmlParserOptions can be passed to xmllib's underlying XML parsing function

The options argument, if provided, is an object whose keys correspond
to flags defined in xml_document.cc. If the corresponding value of the
key is true (or true-like), the flag xmlParserOption flag gets passed
to xmlReadMemory.

Example:

    libxmljs.parseXml(xml, {noblanks: true});

which is particularly useful when calling toString() to produce formatted
output on the document later. If you parse an XML string that has indentation,
insert some nodes, and then try toString() on it, it won't indent the new
nodes properly unless `noblanks` is set in the options.

See:
http://xmlsoft.org/html/libxml-parser.html#xmlParserOption
tests for parseXml options
Merge pull request #225 from JoshData/parseXml_options_argument

add an options argument to parseXml