Add ABNF description of TOML.

One of the most important things we'll need for the TOML 1.0 release is a proper grammar. I've worked one up here as a starting point. I chose RFC 4234 ABNF because it feels much more modern and suitable for a Unicode world than ISO EBNF. Also, JSON uses ABNF and so many people should already be familiar with it.

This grammar should be complete. It also includes scientific notation for floats, as I expect those will go in soon.

I'm certainly no ABNF expert, and I'd love any suggestions for making this grammar more readable.