Rename React to Reactive

To be merged after JuliaLang/React#19 is closed.

Is this the right way to rename a package? Or should the old versions remain in React/ directory and the new one be tagged with a new version in Reactive/? Packages that depend on React will break till someone corrects and tags it... Also If .julia/v0.3/REQUIRE contains React, Pkg.update() will throw an error.