Clarify script_access_type vs script

[stain]

script_access_type feels a bit contrived, as it is defining the type of script key, making it hard to validate/use alone.

The script key however does not explain at all how an inline script can be used.

Other parts of BCO use sub-objects like

 "source": {
    "address": "http://example.com/file.txt"
}

It might make sense to do a similar approach here, where one can provide either address for URI (potentially relative #23 for files) or value for inline scripts (but preferably not both!) - thus one can remove script_access_type

[jpat1546]

Relates to #50

[HadleyKing]

See #50.