Make the application name snake cased when it contains spaces

The application name is used to fill the `database.yml` and
`session_store.rb` files ; previously, if the provided name contained
whitespaces, it led to unexpected names in these files.

Since Shellwords.escape adds backslashes to escape spaces, the app_name
should remove them and replace any space with an underscore (just like
periods previously).

Also improve the assert_file helper to work with paths containing spaces
using String#shellescape.