New methods

Implemented guts(), parent() and matches().  
All tests passing, including t/06_web_query.t (which is my goal)

That said, guts() implementation currently doesn't work with parse_file(), because of a [hackish way](https://github.com/cafe01/html--treebuilder--libxml/blob/master/lib/HTML/TreeBuilder/LibXML.pm#L66) I'm using to know if the parser is adding implicit html and body tags to the tree. Need comments on that.

One way to make parse_file() work with guts() is to read the file content ourself, and then call parse_content(). What do you think?