Documentation overhaul

This is to discuss changes to, and completion of, documentation for this package. Major questions are:
- How do we want to organize the documentation? Formats, locations etc...
- What should be documented (more than just by writing clear code)? Devdocs? Implementation details?
- On what level of detail do we want to document things?

Some discussion started in #45 already, where the following points [were raised](https://github.com/tlycken/Interpolations.jl/pull/45#issuecomment-122535090):

> In a sense the README could be a summary of what's available, now that
>
> ```
> julia> using Interpolations
> julia> ?Interpolations
> ```
>
> dumps the README contents. So I've begun to think that it might be good to keep the README as an "API reminder" for people who already understand the package, and the IJulia notebook as an introduction to the package.

I agree that it's probably a good idea to make the readme minimalistic. I'd also like to start documenting most other things in a `help>`-compatible way, so we might want to move off from the IJulia notebooks alltogether. And I feel like I have to strip down the LaTeX stuff to something with minimal dependencies, so anyone can at least compile it, but it is a very handy way to document the mathematical background of any methods we implement...