More examples in specs (and update to Scala 2.10)

As I am solving these problems, I'm adding more examples to the specs of each problem, in order to better cover the corner cases. I think they are useful for the people starting this now. For now I only have improved specs for Lists and Arithmetic, but I expect to continuously solve the problems in the next one or two months or so.

Every time I thought the behavior wasn't defined in the problem statement (e.g. get the 3rd element of a two-element list), I simply refrained from adding any test at all. Even so, every time I made a test that could possibly be misinterpreted or described a behavior that was't clear in the original problem statement, I added a comment in-place.

Also updated Scala version and dependencies to Scala 2.10 because I was having trouble with the original dependencies. If you want me to submit a pull request with 2.9.2 it's ok. Also added the code of each problem at the beginning of each description just for style preference, but I can also remove them if you don't like.