Fix a typo in README.markdown. s/object/value/
Fix the alignment of an assignment.
s/string/symbols/ in *expected* of `symbols`.

That was already fixed in the name of the parser-producing function
itself but I forgot to change it in the *expected* field.
Add spec-conforming `'expected'` values as per #3.

This involved wrapping several functions in `expect`, as expected. :)
For convenience I've also changed the order of the parameters for the
`success` and `failure` helper functions so the `expected` can be
optional there, covered instead by the `expect` function.