Notify of errors in runner commands

[TylerRick]

Resolves #33

Depends on / based on #27, which aligns things more with the nice, organized way that Airbrake does things in their railtie.

I would have liked to include the full runner command string in the data hash so it's visible in the exception notification. Unfortunately, it appears that ARGV is not available from runner hook, so we can't reconstruct the full runner command string like we did with reconstruct_command_line.

I've worked around this before, like this in the paper_trail-rails gem, but it wasn't as clean as I'd like (basically you have to put your override file at lib/rails/commands/runner/runner_command.rb to get it to be loaded instead of the version from railties).

[TylerRick]

FYI: This PR (and this gem in general) may be redundant now that smartinez87/exception_notification#536 has been merged