[WIP] Add an initial version of an multiple OAuth-Provider example

[wilzbach]

I thought it might be quite worthwhile to try to extract my OAuth code as it's quite different from yours. The main highlights are

• multiple oAuth-providers
• perform a call on the user scope level and retrieve the user object
• persistence for user sessions via mongoDB (I will probably remove this bit as it is easy to do)
• Shows of a couple of cool Vibe.d styles (e.g. with (router), Nullable, ...)
• Loads configs from environment variables (nice for ephermal filesystems like e.g on Heroku)

This depends on #9. This isn't entirely perfect yet, but I ran out of time for this week. However, once #9 is in, I will do a final cleanup on this PR. -> So just see this as a sneak-peek ;-)

[thaven]

Nice example! Multi-provider and use of oauth with application user management are good things to demonstrate. Will certainly merge when it's ready.