Gradle Proof of Concept

Adding ability to run code examples from command line via Gradle

```
$ ./gradlew echoServer
:compileJava UP-TO-DATE
:processResources UP-TO-DATE
:classes UP-TO-DATE
:echoServer
Usage: EchoServer <port>

BUILD SUCCESSFUL

Total time: 0.905 secs
```

```
$ ./gradlew echoServer -Dargs='9090'
:compileJava UP-TO-DATE
:processResources UP-TO-DATE
:classes UP-TO-DATE
:echoServer
com.manning.nettyinaction.chapter2.EchoServer started and listen on /0:0:0:0:0:0:0:0:9090
```

```
$ ./gradlew echoClient
:compileJava UP-TO-DATE
:processResources UP-TO-DATE
:classes UP-TO-DATE
:echoClient
Usage: EchoClient <host> <port>

BUILD SUCCESSFUL

Total time: 4.959 secs
```

```
$ ./gradlew echoClient -Dargs='localhost 9090'
:compileJava UP-TO-DATE
:processResources UP-TO-DATE
:classes UP-TO-DATE
:echoClient
Client received: 4e6574747920726f636b7321

BUILD SUCCESSFUL

Total time: 3.33 secs
```