Move macros to core and deprecate potemkin dep

Based on #146 so please review that first. 

This PR continues that line of simplifying the core.  Commits and changelog are a good summary. Recapped:
 - Prepare to remove `potemkin` as a dependency.  While we will continue to use it internally at Prismatic, there's no reason we need it as a dependency for everyone
      - Deprecate `*use-potemkin*` flag in preparation for removal.  Instead, there will be a different way to stub in a custom `defrecord` constructor if you want it.
      - In a future release, potemkin will be removed as an explicit dependency
 - Stop using `potemkin/import-vars`, and move canonical implementations of `schema.macros` like `defn` to `schema.core`.
       - `import-vars` was causing issues with AOT compilation (see #137), which should hopefully be fixed.
       - The split of `core` and `macros` is no longer necessary since the introduction of `:include-macros` in ClojureScript, leading to a simpler interface
       - Use explicit temporary macro stubs to continue to support deprecated `schema.macros` invocations for now, without using `import-vars`.
 - Increment minimum cljs version to support `:include-macros`