move alt/seq up in the scope
add ::desc to set custom error messages

parser.desc('thing') will yield 'thing' as its expected value in case of
failure.  As a convenience, an optional first argument to `lazy` is the
description of the parser being defined.
add docs for `.desc()` and the extra `lazy` arg
Merge pull request #25 from jneen/feature.desc

Add custom expected values