Node Inspection

This builds on the [packaged library PR](https://github.com/galdor/ruby-gumbo/pull/3), as it renames the ext file to `gumbo_ext` so that requiring `gumbo` gets you both ruby and extension code. Didn't seem to be worth trying to separate those pull requests :(

* Adds support for `original_end_tag`
* Adds `_range` helpers to various node types
* Adds `to_s` to spit out original text where reasonable
* Modified the class hierarchy to match the DOM hierarchy (Comment,CData,Whitespace are subclasses of Text)