Merge 'origin/master' into retreat-slashing

# Conflicts:
#	node/cli/src/chain_spec.rs
#	node/runtime/src/lib.rs
#	srml/aura/Cargo.toml
#	srml/babe/Cargo.toml
#	srml/babe/src/lib.rs
#	srml/session/src/lib.rs