Fix decoding of quoted keys

This removes a few issues related to quoted keys:

- Escape sequences now work in table headers and KeyValue keys.
- Empty keys are allowed when quoted.
- The decoder doesn't see the quotes anymore. This already worked
  for (inline) tables because the parser split the key manually after
  decoding, removing quotes. The new version integrates the splitting
  into the grammar so there is only one place that handles keys.

Fixes #37

fixed typo: 'themshelves' -> 'themselves'

Closes #9