Allow configuration of redirect uri in the client

[jgannonjr]

A redirect uri can now be passed in the options in the configure method:

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.

[shayne]

@jgannonjr mind rebasing?

[jgannonjr]

Sorry Shayne, for some reason I totally missed your comment and just happened to check back in on this thread. Anyways, I rebased on the current master, cleaned up the added tests, and moved the changes into a topic branch. Github won't let me change the target branch of this pull request, so I had to make a new one, which is #88.