Introduce basic support for nanoc environment:
- nanoc command is updated to include --env argument
- config are overriden using the active environment if any
- unless define in environment, output_dir is {{output_dir}}/{{env_name}}

Setting environments is done in nanoc.yaml using the `environments` property.
Example usage is:

```
output_dir: output

environments:
  default: &default
    base_url: ...
    ...
  development:
    <<: *default
    base_url: ...
  production:
    <<: *default
    base_url: ...
  yet_another_env:
    <<: *default
    base_url: ...
    output_dir: build
```

Selecting working environment can be done:
- using environment variable `NANOC_ENVIRONMENT`
- using `nanoc --env=[<value>]`

- Document env attr_reader
- Only allow String and nil for environment name
- Remove argument for with_environment
- Make :environments a constant
- Lot of parentheses fix
- Renamed NANOC_ENVIRONMENT to NANOC_ENV for consistency with Rails, Rake, etc.
- Prefer Fetch(a,b) over a||b
- Refactor tmp_path logic into Nanoc::Int::Store
- Make -e, --env argument required
- Updated test according to previous changes

- add Contracts
- rename store to store_name
- use named arguments

Support nanoc environments