Add text nodes

[remysucre]

This PR adds text content as children nodes to the parent node, e.g. <p>Hello<br>World</p> will parse to a p node with 3 children: a text node containing Hello, the br node, and a text node containing World. A text node has nil as its tagname (because it has no tagname), but we can discuss this choice. The added and modified tests give some sense of the new behavior.

This is similar to #49 with a few differences:

1. Text nodes have nil as tagname which will never create collisions
2. This PR supports multiple text nodes intermixed with nested tags, like <p>Hello<br>World</p>

[remysucre]

6e41361 uses _text instead of nil as the tagname for text nodes, so that they can be selected with selectors (but may create conflicts if there are also <_text> tags).
For clarity I also renamed the existing _text field to _rawtext and gettext() to getrawtext(), but that is not necessary because text nodes are added as elements, not a field.

[msva]

Well, unfortunatelly, I have almost no time to properly test that all, but at quick look, it looks very good.

Although, it would be nice to add some warning to readme about (potentially) breaking change in API.

If I'll be lucky, I'll try to merge this at weekend (but can't promise)