Deprecate `normalized_text` method, replace it with `text` property.

The `text` property is more intuitive and easier to remember.
The `text` property has almost the same result as `normalized_text`,
but it represents `<br/>` and `<p>` with single and double newlines respectively.
`text` is to be the lxml `text` property, which contained the raw, non-recursive
text of the current node and is now available as `raw_text` property.