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", ExecResultEntry gives access to exec_context fields.
Added "exec_" as an alias to "exec_event": it's shorter, distorts original name less and follows PEP-8 recommendation.

Added __repr__ method to elliptics.Id (prints group, id prefix, original string).
Modified elliptics.Id.__str__ to always return entire id instead of 6 byte prefix.