Update README and Example

It seems to me that the README and the Example app aren't up to date with the latest API. Firstly this pull changes the dataset definition from an atom to a string otherwise Elixir fails to compile and throws the following error:

```
** (FunctionClauseError) no function clause matching in String.Unicode.next_grapheme/1
    /private/tmp/elixir-4KO4/lib/elixir/priv/unicode.ex:171: String.Unicode.next_grapheme(:weather)
    /private/tmp/elixir-4KO4/lib/elixir/lib/string.ex:669: String.first/1
    /Users/Tristan/Desktop/ecto/lib/ecto/adapters/postgres/sql.ex:317: Ecto.Adapters.Postgres.SQL."-create_names/1-fun-0-"/2
    /private/tmp/elixir-4KO4/lib/elixir/lib/enum.ex:1522: Enumerable.List.reduce/3
    /Users/Tristan/Desktop/ecto/lib/ecto/adapters/postgres/sql.ex:316: Ecto.Adapters.Postgres.SQL.create_names/1
    /Users/Tristan/Desktop/ecto/lib/ecto/adapters/postgres/sql.ex:27: Ecto.Adapters.Postgres.SQL.select/1
    /Users/Tristan/Desktop/ecto/lib/ecto/adapters/postgres.ex:39: Ecto.Adapters.Postgres.all/2
    /Users/Tristan/Desktop/ecto/lib/ecto/repo.ex:231: Ecto.Repo.all/3
```

Also `fetch` no longer exists and the equivalent appears to be `all` now.