Store provenance in crates about the tool which created it

[Pfeil]

Problem statement:

It is not possible to understand if a crate was created with ro-crate-java (and which version of it) or by which software it was implemented.

Suggestions:

• document that ro-crate-java created or modified a crate (with timestamp etc) within the crate itself
• allow developers to easily extend this information (e.g. about which software was it that used ro-crate-java to create the crate?)

Probably the best way is to provide a specialized context entity or so. There should be a minimum added by default, and it should be easy to add more such information.

[Pfeil]

Merged into development branch for the next release (candidate).