examples/priv: Use new elliptic_curve_key.

The way ECK is used has changed a bit, so accomodate the example.

Announcement: http://lists.dyne.org/lurker/message/20140116.144459.92a367f2.en.html

Issue: https://github.com/spesmilo/libbitcoin/issues/33
Merge pull request #34 from veox/issue33

examples/priv: Use new elliptic_curve_key (fix issue #33)