Bug fix: Event returns Selection module

Will cause things to go wrong if someone initializes the Event module after Selection has already been initialized.
Merge pull request #9 from evancz/patch-2

Bug fix: Event returns Selection module
Fix up top-level description

The previous description wasn't that descriptive or coherent. This is an
incremental improvement.

Closes #8.
Build with elm-get and Elm compiler

Switch to Elm’s native module format for cast.js and gensym.js. This
makes it possible to bundle everything up with elm-get and post to
library.elm-lang.org.
Move down a directory to work with elm-get

I need to add a src-dir field to elm_dependencies.json files, but have
not been able to yet. Until that happens, that means stuff needs to be
top level to be compatible.
Remove Makefile, which is no longer necessary

Because things are bundled by elm-get, these files will be created and
included automatically.
Add/format docs to work with elm-get

I tried to make the flow of the docs reasonable for a new person such
as myself, but of course we can continue to revise. With these changes,
docs can be uploaded to library.elm-lang.org
Add Cast and Gensym to native-modules
Correct syntax error and missing id functions

When I converted things to work with elm-d3, I missed a few changes.
Merge branch 'elm-get' of https://github.com/evancz/elm-d3 into elm-get