Add HTML parser supports

Allow RXMLElement parse HTML. Depends on ```libxml2/libxml/HTMLparser.h```

Add following constructors:

    - (id)initFromHTMLString:(NSString *)xmlString encoding:(NSStringEncoding)encoding;
    - (id)initFromHTMLFile:(NSString *)filename;
    - (id)initFromHTMLFile:(NSString *)filename fileExtension:(NSString*)extension;
    - (id)initFromHTMLFilePath:(NSString *)fullPath;
    - (id)initFromHTMLData:(NSData *)data;
    + (id)elementFromHTMLString:(NSString *)xmlString encoding:(NSStringEncoding)encoding;
    + (id)elementFromHTMLFile:(NSString *)filename;
    + (id)elementFromHTMLFile:(NSString *)filename fileExtension:(NSString*)extension;
    + (id)elementFromHTMLFilePath:(NSString *)fullPath;
    + (id)elementFromHTMLData:(NSData *)data;