Add support for encapsulated models

[felix]

PR #59 adds support for encapsulated models in bit strings or octet strings.

Examples can be found in RFC 5280:

Extension  ::=  SEQUENCE  {
        -- ...
        extnValue   OCTET STRING
                    -- contains the DER encoding of an ASN.1 value
                    -- corresponding to the extension type identified
                    -- by extnID
        }

or in RFC 5912 where it is explicitly used:

Extension{EXTENSION:ExtensionSet} ::= SEQUENCE {
     -- ...
      extnValue   OCTET STRING (CONTAINING
                  EXTENSION.&ExtnType({ExtensionSet}{@extnID}))
                  --  contains the DER encoding of the ASN.1 value
                  --  corresponding to the extension type identified
                  --  by extnID
  }

[indutny]

I guess this can be closed now?

[felix]

Added in a2d711e