Proper support for session::exec in python binding

Added support for src_key.
src_key and data args made optional.
Added async variant "exec_async" and ExecResultEntry with gives access
to all data from exec_context.
Added "exec_" as an alias to "exec_event": it's shorter, distorts
original name less and follows PEP-8 recommendation.

Also added representation methods for elliptics.Id (__str__ and __repr__).