Add decryption functions and attempt decrypt if pdf is encrypted

[prscoelho]

Fix #37 based on #59

This change adds extract_text_encrypted, extract_text_from_mem_encrypted and output_doc_encrypted.

Try to decrypt with empty password before calling output_doc and return early with error if encrypt("") returns an error.

output_doc still has a is_encrypted() check, as lopdf removes the Encrypt trailer after successfully decrypting.