Feature complete

[christian-schlichtherle]

• Adds LDAP support (optional)
• Introduces KksIdentity interface to resolve the user's private key and certificate.
• Provides KeyStoreIdentity implementation.
• Introduces KksDirectory interface to resolve second party certificates.
• Provides KeyStoreDirectory implementation.
• Provides LdapDirectory implementation.
• Updates KKS facade class to reflect API improvements.
• Updates Main class to enable using the new features.
• Updates Javadoc all over the place.
• Adds test cases for LDAP operations.