Allow configuration of redirect uri in client

*NOTE:  The description below is copied from https://github.com/Instagram/instagram-ruby-gem/pull/49.  I cleaned up and rebased https://github.com/Instagram/instagram-ruby-gem/pull/49 on the current master and moved the changes into a topic branch.  Github won't let me change the branch target of an open pull request so I had to make a new pull request.*

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.