Make find-extra-keys-schema public

For coercions on map-schemas `find-extra-keys-schema` can be useful, so this patch makes it public.

As proposed by @w01fe in https://groups.google.com/forum/#!topic/prismatic-plumbing/SaOBraHzoHE
Merge pull request #136 from abp/patch-1

Make find-extra-keys-schema public