0.18 upgrade

[john-kelly]

• [x] use new elm-lang/http
• [x] in' -> inList
• [x] not' -> not
• [x] update examples with new API
• [x] improve relation api
• [x] include is not Maybe
• [x] elm-package.json of /
• [x] elm-package.json of /examples
• [x] nullable fields helper
• [x] opaque limit api
• [ ] elm-package.json of /test (need to wait for elm-test and node-test-runner to be updated)

[john-kelly]

fixes https://github.com/john-kelly/elm-postgrest/issues/11

[john-kelly]

fixes https://github.com/john-kelly/elm-postgrest/issues/15

[john-kelly]

fixes https://github.com/john-kelly/elm-postgrest/issues/13