Introduce Runtime Events (#607)

* Squashed commit.

New slashing mechanism (#554)  …
* Slashing improvements

- unstake when balance too low
- unstake after N slashes according to val prefs
- don't early-terminate session/era unless unstaked
- offline grace period before punishment
* Fix warning
* Cleanups and ensure slash_count decays
* Bump authoring version and introduce needed authoring stub
* Rename
* Fix offline tracker
* Fix offline tracker
* Renames
* Add test
* Tests
* Tests.
Remove accidental merge files.
Merge remote-tracking branch 'origin/master' into gav-new-pos
Version bump, fixes (#572)  …
* Bump version, don't propose invalid blocks
* Fix build.
* Fixes.
* More fixes.
* Fix tests.
* Fix more tests
* More tests fixed
Fix merge
Fix accidental merge bug
Fixes.
Staking failsafes  …
- Don't slash/unstake/change session when too few staking participants
- Introduce set_balance PrivCall
Make minimum validator count dynamic.
test fixes
Fix tests.
Fix tests
Fix tests, update readme.
Merge remote-tracking branch 'origin/master' into gav-new-pos
Test with release.
Use safe math when dealing with total stake
Fix test again.
Introduce events into runtime.
Fix tests
Add events for account new/reap
Integration-style tests for events.

* Remove old code

substrate/demo/executor/Cargo.toml

@@ -20,6 +20,7 @@ demo-runtime = { path = "../runtime" }
 [dev-dependencies]
 substrate-keyring = { path = "../../substrate/keyring" }
 substrate-runtime-primitives = { path = "../../substrate/runtime/primitives" }
+substrate-runtime-session = { path = "../../substrate/runtime/session" }
 substrate-runtime-staking = { path = "../../substrate/runtime/staking" }
 substrate-runtime-system = { path = "../../substrate/runtime/system" }
 substrate-runtime-consensus = { path = "../../substrate/runtime/consensus" }