Add support for import/export of DSA keys

*NOTE: This pull request builds on top of the existing pull request #32 (PKCS#8).*

This extension adds support for import/export of DSA keys.
As for RSA keys, there are several formats available:
* DER (binary) vs PEM (text) vs OpenSSH (text)
* X.509 vs PKCS#8 vs OpenSSL/OpenSSH
* Encrypted vs clear (all encryption algorithms introduced by the previous PKCS#8 pull request for RSA are automatically supported for DSA too)