Allow configuration of redirect uri in the client

A redirect uri can now be passed in the options in the configure method:

```ruby
Instagram.configure do |config|
  config.client_id = 'CI'
  config.client_secret = 'CS'
  config.redirect_uri = 'http://localhost:3456/oauth/callback'
end 
```

This will be used as the default `redirect_uri`.  `authorize_url` and `get_access_token` no longer need to be passed a `:redirect_uri` option, however that option can still be passed to override the configured `redirect_uri` if one has been specified.