support AES-256 encryption

[fredmafu]

New const of encryption type like PdfWriter.ENCRYPTION_AES_256.

Thank you.

[andreasrosdal]

Pull requests welcome!

[Lonzak]

We can not take AGPL code and put it into this LGPL/MPL licensed code.
There is an development in this area however and an independent implementation of this.
Something is soon to come :-)

[bsanchezb]

Hello!

I'm also interested in this and support of revision of the standard security handler with number "6" in particular (/Encrypt dictionary with attribute /R 6).

@Lonzak, do you have any updates on it?

Best regards,
Aleksandr.

[Lonzak]

@bsanchezb Yes the update is that it has been implemented :-)

[bsanchezb]

@Lonzak , thank you for the notification ;) and big thanks to @mkl-public for the PR