Fix : only one iterator per context

Fix : only one iterator per context

As of now, the API let the user create multiple iterators on the same
context but the underlying code was not ready. This patch restrict to
only one iterator per context. Supporting multiple iterators is
targetted for the next release.

Fixes #166

[ Edit by Mathieu Desnoyers: update API comment ]

Signed-off-by: Julien Desfossez <julien.desfossez@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>