Issue #1: Applicative/Monadic validation.

[rpiaggio]

Here's the first iteration to adapt the library to different contexts with a minimal change in the API.

Please see the tests in ContextSpec.scala.

I don't consider this PR ready for merging yet.

There are a few things where I find myself in need or help, or that are worth discussing:

• Kleisli and Profunctor integration (for the moment they are commented out).
• Rule Applicative laws testing. Currently commented out. If uncommented, some implicit(s) are missing. Without delving further into laws testing, I'm currently unable to sort this out.
• Due to their new shapes, I am currently falling short of finding a satisfactory way of letting Rule1.field (and .fieldWith) take a 2nd Rule1 as an implicit parameter. Everything works fine with an explicit 2nd Rule1.
• I added a Rule1.and method. The difference with andThen is that it requires the Rule1s to be just Applicative instead of Monad. This makes it possible to run the rules in parallel whenever their context's Applicatives define parallel execution (as is the case of, say, Future). However, it is not right to define this for every Rule1, since a Rule1 can return the same type as the input, but another value. I think we actually need a subclass of Rule1 (which I would call Check) whose semantics indicate that it's a Rule1 that always returns the same value as the input. (Rule.test would return Check). Combining Checks with and results in a Check, and they can be combined with other Rules by using andThen (resulting in a Rule). For the moment Check is just a Rule1.