Add config setters

[jniesen]

I wanted a way to configure the plugin from a Groovy script. Having this would make allow me to create a Groovy init script for use with the official Jenkins Docker image.

The changes included in these commits will allow you to configure the plugin like so:

import jenkins.model.*
import org.datadog.jenkins.plugins.datadog.DatadogBuildListener

def j = Jenkins.getInstance()
def d = j.getDescriptor("org.datadog.jenkins.plugins.datadog.DatadogBuildListener")
d.setHostname('https://foo.bar.com')
d.setTagNode(true)
d.setApiKey('abc123')
d.setBlacklist('job1,job2,,job3 , job4')
d.save()

[JohnLZeller]

Hey @jniesen, great idea! I'll take a look at this, and get you some feedback :) Thanks again

[JohnLZeller]

Rebased here: #64