TASK: GraylogLogger should use new `logThrowable()` if possible

TASK: Document new message context feature

TASK: Add Flow 5.0 compatibility

TASK: Update license header

Merge pull request #14 from johannessteu/logger

FEATURE: Add new GraylogLogger

Merge pull request #9 from robertlemke/multilog

FEATURE: Log all messages to System Logger as well

Merge pull request #10 from robertlemke/fix-flow-dependencies

Fix Flow dependencies

TASK: Make sure that the GraylogService is present

FEATURE: Tweak README

BUGFIX: Add missing `Flow\Inject` annotation