Minor fixes & improvements

I packaged Tutter as a Rack compatible app (runs on Heroku and Dokku):
https://github.com/jpettersson/tutter-rack

While working on this I found a few issues and minor improvements that I address in this PR. None of the changes should break the current API and the specs pass.

* Upgraded to latest Octokit: I ran in to strange 404 errors with the old one.
* The sinatra-reloader gem was missing from the Gemfile, added it.
* Added the ability to specify the config file path with a ENV var.
* Added the ability to keep access tokens in ENV variables and point to them from the config.
* Checking for valid request based on the x-github-request header. Properly respond to the 'ping' event after hook setup.
* Passing the x-github-event value to Action instances: instead of having to check fields in the data to find out what kind of event happened.

Now I'm going to start on the autorelease plugin :)