Create dist directory in BuildFromSource

In preparation of #19, I thought it might be useful to bundle all resources that are necessary for a working installation into one directory. Being a total Haskell noob, there are probably way nicer ways of achieving the same thing, but I wanted to know first whether you think that this is reasonable approach in general.

If you set your `ELM_HOME` to `$INSTALL_DIR/Elm-Platform/0.13/dist/share` and add the `bin` to your `$PATH` respectively, you can directly use it.