v0.6.0

This release brings significant speed increases (>95% in some places) to the compositing step, courtesy of @bend-n and his fimg library.

• If you build from source - please note that the nightly compiler is now used instead of stable. cargo should automatically pick up on this, but it's something to be aware of regardless.
• If you use the provided binaries, please be aware that they now expect a CPU supporting SSE 4.1 (i.e. any remotely modern chip.)