"Slave polling hangup" issue workaround (JENKINS-15315)

Dear Robert,

I've added configurable timeout to com.tek42.perforce.parse.AbstractPerforceTemplate::getPerforceResponse().
It provides workaround for [JENKINS-15315][1].

Timeout can be configured via global config. By default, timeout is disabled, so plugin will use legacy BufferedReader::readLine() approach. In addition, plugin has minimal timeout requirement (30 seconds)

![config_ex](https://f.cloud.github.com/assets/3000480/670770/3649a830-d84a-11e2-8e3a-113a9c0dbae6.png)

Best regards,
Oleg Nenashev
R&D Engineer, Synopsys Inc.
www.synopsys.com

P.S: I've modified Executor's interface in order to extract status of external process (added isAlive() method). BTW, I haven't tested this method's implementation for CmdLineExecutor.

[1]: https://issues.jenkins-ci.org/browse/JENKINS-15315