ability to use Executor instead of ThreadFactory

Netty is widely used in IntelliJ IDEA — java compiler server, built-in server (xmlrpc, json), built-in web server, js debuggers and so on. I have tried to migrate to Netty 4, but encountered a problem — we cannot use now our shared (application level) executor service. We cannot use thread factory — application doesn't expose it and you cannot create threads, you must use shared pool of threads (exposed as executor service).

Please review my pull request:
1) Previous public API is not removed, you can use thread factory as before.
2) All tests passed. Code (SingleThreadEventExecutor:780)
```
if (interrupted) {
  thread.interrupt();
}
```
may be is not good solution (without it test SingleThreadEventLoopTest.scheduleLaggyTaskAtFixedRateB will be failed) but all works and I have tried to minimise my changes.