#505, Allow custom Flapjack branding.

This is to deal with #505, Allow custom Flapjack branding.

The change allows for an image_logo_path setting in the config that points to a custom logo file. 

I'm not 100% sure if this PR should be considered. I'm unhappy with it for two reasons:

1. The logo is copied in from the path into a /public/img/custom folder (created when the Sintra app starts up). This feels like a hack. If I'd started this again I would have used sendfile from the image's default location, though this might have caused performance / security issues. 

2. I had to comment out a test that works in isolation (defaulting to the standard logo if nothing configured). The test will sometimes fail, possibly if the primary spec for the logo executes first and the @config variable gets set. I can't bend my head around this - the start method should load the config, and it's being called independently in each of the tests. There's some weird interdependence between specs in the web_spec.rb file, but I can't for the life of me figure it out - at least not in the time available.

Any feedback appreciated. 

Rather than start with `if !` it might be good to invert the clauses -- a matter of taste though.