v0.3.0

Changes in this release:

• Fix bugs with string handling for information coming from library, slot and token
• Add certificate, key and signature interchange functions for X.509 and OpenSSL
• Fix bugs/add tests when exchanging data with OpenSSL/Python crypto libraries
• Cache object properties
• Fix encoding of EC_POINT for derive_key
• Use AES_MAC as the default signing/verification mechanism for AES
• Fix some crashes, e.g. when no mechanisms are present