Use a forked exec-maven-plugin instead of maven-antrun-plugin

Motivation:

maven-antrun-plugin does not redirect stdin, and thus it's impossible to
run interactive examples such as securechat-client and telnet-client.
org.codehaus.mojo:exec-maven-plugin redirects stdin, but it buffers
stdout and stderr, and thus an application output is not flushed timely.

Modifications:

Deploy a forked version of exec-maven-plugin which flushes output
buffers in a timely manner.

Result:

Interactive examples work.  Launches faster than maven-antrun-plugin.