v0.5.0

Mostly an hotfix release for #22.

New

• New MQTT plugin.

Fixes

b420615 fix: start reporting stats when session is fully initialized (fixes #21)
f71ddb0 fix: Dockerfile compiled twice
747fcfd fix: check if expression is a file other than filesystem existance (fixes #22)
ef93650 fix: permafix for #8 without build.rs
9b1b582 cargo fix

Various other fixes and refactorings.