…e list of acceptable headers.

getFromHeader will return null if the accept header is either missing, or set to accept everything. This will allow the default to be used instead.

…ities.

Remove the priorities member variable and functionality.

…fore we no longer need to check for "all" in getFromHeader.

If the user explicitly sets default format to be null, then the system will generate a 406 Not Acceptable response if no types match. For example:

```
//Code
Middleware::FormatNegotiator([
    'json' => [['json'], ['application/json', 'text/json', 'application/x-json']],
    'zip' => [['zip'], ['application/zip', 'application/x-zip', 'application/x-zip-compressed']]
])
->defaultFormat(null)

//Header:
Accept: application/pdf

//Result
406 Not Acceptable
```

Readme updated to reflect.