Added support for data encoding

In response to issues #4 and #9. Added private NSString variable for
encoding, the corresponding readonly property and miscellaneous
initializers. Added PerformHTMLXPathQueryWithEncoding and
PerformXMLXPathQueryWithEncoding to maintain the current interface /
backwards compatibility.
Merge pull request #28 from nothine/master

Added support for data encoding
Merging latest changes from upstream.