oox: add Digest class which uses NSS or OpenSSL for digest calc.

Document encryption and decryption uses either NSS or OpenSSL to
calculate digest. Digest class hides the implementation details
between the two implementations. Previously, functions sha1 and
sha512 were used for this, but were less generic.

Change-Id: I60119e2ab9c5c1f4a2b02bc417c3c89c53a63fda