+ can: add `spray.can.client/server.incoming-auto-chunking-threshold-size`, fixes #189

The setting is still disabled per default because it is not yet clear
how the higher-level API are going to deal with it generally.
= can: add streaming file upload code to simple-http-server example

This is the most basic form working with the current version of the
chunking message protocol. Once we have implemented registering for
chunked messages the `chunkHandlers` map can go.
= can: always produce chunks if Content-Length >= autoChunkingThreshold

I.e. also when the complete body is already available. Thanks, Bob Ngu for
the fix.
Merge pull request #425 from jrudolph/wip/chunkless-request-streaming

+ can: add `spray.can.client/server.incoming-auto-chunking-threshold-size`, fixes #189