Alias to_xml methods as to_html

Here is one addition up for discussion: Alias the `#to_xml` methods as `#to_html` 

This at least for me makes the api more usable and consistent. eg. writing
```ruby
Oga.parse_html("<html><body></body></html>").to_html
```
Instead of 
```ruby
Oga.parse_html("<html><body></body></html>").to_xml
```

I don't know if this is the best way to implement this, but since both xml and html use the same parser, a simple alias seemed the cleanest solution.

Comments, discussion?