Support for namespaced XPath queries

Allows the user to supply a ```namespaces``` option. ```namespaces``` must be an object of identifier => namespace key/value pairs, in the same format expected by [xpath.useNamespaces](https://www.npmjs.org/package/xpath#namespaces-with-easy-mappings). The ```namespaces``` value is then passed directly to [xpath.useNamespaces](https://www.npmjs.org/package/xpath#namespaces-with-easy-mappings).

This allows xmlpoke to perform xpath queries on XML documents with namespaces.  

xpath version was upgraded from v0.0.5 to v0.0.6, which provides the [useNamespaces](https://www.npmjs.org/package/xpath#namespaces-with-easy-mappings) method.

Example usage:
XML Document:
```
<?xml version="1.0"?>
<RDF xmlns="http://www.w3.org/1999/02/22-rdf-syntax-ns#" xmlns:em="http://www.mozilla.org/2004/em-rdf#">
	<Description about="urn:mozilla:install-manifest">
		<em:version>1.2.3</em:version>
	</Description>
</RDF>
```
Gruntfile.js
```
    xmlpoke: {
      updateFirefoxVersion: {
        dest: 'destination.xml',
        src: 'source.xml',
        options: {
          namespaces: {
            'em': 'http://www.mozilla.org/2004/em-rdf#'
          },
          xpath: '/RDF/Description/em:version',
          value: '1.2.4'
        }
      }
    }
```