Add support for using NSData for passwords/keys.

When using some algorithms like diffie-hellman the resulting key is not
a printable string.
This change adds support for storing that kind of keys.
Merge branch 'master' of https://github.com/dcu/sskeychain into test-merge

# By David A. Cuadrado
# Via David A. Cuadrado
* 'master' of https://github.com/dcu/sskeychain:
  Add support for using NSData for passwords/keys.