Nice initiative.
I didnt decided about DB server yet.
MongoDb is not bad.Also depends on what language you're using as middleware.I suggest checking out Couch/Pouch Db.I didn't see no running instructions at the moment.Does it work or anyway to run it locally ?
Go is a descent choice but scala would do wonder in the stack you have defined.TDD based development gives me organisms.I wish I could contribute but I'm not familiar with GO and don't have much time to invest into learning it.
MongoDB is good as starter kit. Easy to deploy single master. But if application matures enough to deploy it to AWS, MongoDB becomes problem, it wont replicate properly over availability zones and all this Master Slave thing is annoying. Will try to abstract it somehow. I personally hate DBs and try to push it as far as i can
Dont be afraid of GO. Seriously its like C with everything your ever wanted to change in it. Its minimalistic fast and straight to the point. Dependency management works like a charm it runs on top of github so no need for artifactory. Static types. There are libraries for everything you want. No Java bullshit with Springs dependency injections and configuration files for every little thing you want to do.
Scala is awesome. No doubt. I say no to scala because of one thing - JVM. Its real pain in the ass to manage java versions on production servers. Yes i know i can "compile it" it , or make war file , or bundle it, or whatever fancy trick is in the hype this month. And you need artifactory after some time....maven. gulp. cradle...well you get my point.
With go you just copy paste your binary and you are done. It will work as long as processor instruction set is not changed. And its not some fancy packaging system that "will solve all your problems" , its core feature of the language itself.