Change folder structure of `smpl create`

How about putting all source files in a `src` directory and change `Application.hs` to `MyAppName.hs` within the `src` directory? Or `app` instead of `src` is also a good option.

For example `smpl create test_blog` would create the following structure:
```
layouts/
views/
src/TestBlog/Common.hs
src/TestBlog.hs
Main.hs
test_blog.cabal
```
