#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. 

For a single clause, yes,, but as this has branches for either case `unless ... else ...` is not very nice. As `@logo_image_url` is set by multiple branches here I'd possibly make that value the result of all of them, e.g.

```ruby
default_logo_image_url = "#{@base_url}img/flapjack-2013-notext-transparent-300-300.png"

@logo_image_url = if logo_image_path
  if File.file?(logo_image_path)
    # snip...
    "#{@base_url}img/custom/#{logo_image_filename}"
  else
    default_logo_image_url
  end
else
  default_logo_image_url
end
```