Replace Finatra 1.6 with finagle-http, fix quick start and examples

[travisbrown]

This PR includes @eightysteele's changes from #95 but rebased and with a few little fixes for recent updates.

It also replaces brushfire-finatra with brushfire-finagle, which does more or less the same thing but without the dependency on the EOL-ed Finatra 1 (which won't be published for 2.12). Since this is just an example project (that wasn't compiling anyway) I figured a pretty major change wouldn't be a problem. We could also just delete the dead code, but it seems like a nice thing to have.

I've added an ./iris-srv example to the README, since it works again. I'd like to aggregate the module in the build, so that we know when it breaks, but because the latest Finagle version isn't built for 2.10 this causes problems for ++ and + (we could use sbt-doge to fix this but in my experience it's a pain and in this case probably isn't worth the effort).

More generally it seems like it'd be a good idea to pull all of the example code into a new brushfire-example module, so that stuff like local.Example isn't ending up in our published artifacts. If people agree I could do that in this PR, or in a follow-up.

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