Fix items validator.

Make the "items" keyword unmarshal successfully if its value is a valid
schema. Before it would only unmarshal it its value was a list of
schemas (both are valid values for "items").
Improve style.
Merge pull request #28 from cupcake/fix-items-val

Fix items validator.
Schemas store validators as map[string]Validator.

The old way had schemas store validators as []Validator. The new way is
better because the strings (which are validator names like "maximum")
are needed for resolving references to internal schemas.
Current WIP.