markdown cleanup, check different locations for config, notes on running as upstart service

I'm very happy to have found graphital. Thanks!

The code changes in this pull request mean that graphital can be installed once, and then used with different config and a different `graphital.d` dir, without changing anything in the original install location.

No present functionality is affected.

It resolves the concern of having to choose between the not quite ideal options of:
* Clone to install, but manually change the working copy for desired config and checks.
* Maintain config and checks in a fork that can't be rebased when updates come to the official repo.
* Pull `graphital.rb` itself, as well as custom config and checks, into another repo of general infrastructure code.