Add ClojureScript support via cljx

*   Migrate everything that's non-JVM specific to be shared via cljx
*   Update project.clj
    *   Bump schema dependency version
    *   Add cljx configuration
    *   Add cljsbuild configuration
*   Add plumbing.graph-async namespace. See notes in CHANGELOG
*   Misc .gitignore updates
Merge pull request #30 from Prismatic/v0.3.0

Add ClojureScript support via cljx