Rewrite the actor, the i/f become simple and clean and without OO.

- All methods contained in a table.
  - `create` - create an actor
  - `start` - start an actor
  - `register_event` - register an event
  - `unregister_event` - unregister an event
  - `send` - send a message to another actor
  - `wait` - wait a message
  - `run` - run
- Use environ-var LUACTOR_REACTOR to choose the reactor.
- README.md is fully written by english now.
- actor.lua -> luactor

Signed-off-by: Xiongfei Guo <xfguo@credosemi.com>