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 print entire id bytes.