Bump to 0.3

* Bugfix: the signature could sometimes be unterminated. After
  gpgme_data_read(3) returns 0 bytes, terminate the buffer string with
  \0.

* Bugfix: catch missing private keys. If you pass a keyid and the keyid is
  not that of a secret key then it will not set the key. In this case it
  checks it against NULL. However, `key` must be NULL initially for this
  to work.

* Support offline master keys. We previously searched for secret keys, which
  ignores secret subkeys. Instead we now search all keys in the keyring,
  skipping those that cannot sign (also those that are revoked, expired,
  disabled, or invalid). The result is that the user can specify the
  master keyid even if their master key is offline but they have a
  usable signing subkey.