[issue #3] Add an option to skip the auth system.

The `IZANAMI_AUTH` environment variable is added to skip the built in
HTTP Basic Auth system provided by `rack`.

Pass `IZANAMI_AUTH = disable` to skip all authentication, or `enable` to
use it. When no value is passed, `enable` is the deffault.
Merge pull request #4 from wuakitv/disable-auth

[issue #3] Add an option to skip the auth system.