Expose lmdb::Iter publicly (#8)

I'm trying to make a newtype wrapper around it, and can't because
it's in `lmdb::cursor`, which isn't public.

src/lib.rs

@@ -14,7 +14,8 @@ extern crate lmdb_sys as ffi;
 pub use cursor::{
     Cursor,
     RoCursor,
-    RwCursor
+    RwCursor,
+    Iter
 };
 pub use database::Database;
 pub use environment::{Environment, EnvironmentBuilder};