Added test for multiple entry sequence
Start making the grammar more real.

I added obvious YAML grammar stuff. The tests still pass but now lots of
uncovered grammar rules. Need to add tests for them.

Started documenting the grammar rules. This will be important for others
to understand what's going on.

Got a basic idea of where indent / ondent / undent go, but they are
no-op rules so far.
Got flow mappings work. Ton of refactoring.
Add support for flow sequence and...

get the receiver working more correctly.
Add block and flow flags to collection events.