[Scala/en] Reorganize the Scala tutorial

Adam,

This PR is gonna be a confusing one. What I did was I first wrote out an outline of the major topics of Scala I think should be covered, ordered so that we can minimize forward dependencies. Then I went through the old tutorial, and moved each piece under the appropriate heading. This way we get some structure to the page, and we can look at each section and improve them separately.

A few obvious flaws that fell out of this re-arrangement:

- There were two separate sections on strings
- The "def" function syntax is missing entirely (except briefly mentioned in the OOP section)
- No mention of the "return" statement and how it differs from traditional languages
- No mention of implicits

Anyway, I'm sure there are lots more to improve, but just illustrating the point that we really need some structure to this page. Anyway, take a look, I also put in a few minor fixes.

Geoff