Add gradle-info metadata to pom files

Add a new plugin (nebula-metadata) that will pull data from gradle-info and insert the key/value pairs into the published pom file's <properties/> node.  The key's are prefixed with 'nebula.' to avoid collision and confusion with an existing properties.

The particular implement is the simplest path, but also likely the least desirable.  It results in a pom file that has a bunch of properties that may look foreign to somebody accustomed to maven pom's.  An alternative approach would be to add a new namespace (assuming that is possible) to the pom.xml.

Here's an example of the resulting pom file.
```
<?xml version="1.0" encoding="UTF-8"?>
<project xmlns="http://maven.apache.org/POM/4.0.0" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <modelVersion>4.0.0</modelVersion>
  <groupId>nebula.hello</groupId>
  <artifactId>world</artifactId>
  <version>1.0</version>
  <dependencies>
    <dependency>
      <groupId>asm</groupId>
      <artifactId>asm</artifactId>
      <version>3.1</version>
      <scope>runtime</scope>
    </dependency>
  </dependencies>
  <properties>
    <nebula.Manifest-Version>1.0</nebula.Manifest-Version>
    <nebula.Implementation-Title>nebula.hello#world;1.0</nebula.Implementation-Title>
    <nebula.Implementation-Version>1.0</nebula.Implementation-Version>
    <nebula.Built-Status>integration</nebula.Built-Status>
    <nebula.Built-By>mmcgarr</nebula.Built-By>
    <nebula.Build-Date>2014-07-03_15:33:43</nebula.Build-Date>
    <nebula.Gradle-Version>1.12-20140608201532+0000</nebula.Gradle-Version>
    <nebula.Module-Source>/build/test/nebula.plugin.responsible.NebulaMetadataPluginLauncherSpec/published-pom-contains-a-collected-property</nebula.Module-Source>
    <nebula.Module-Origin>git@github.com:nebula-plugins/nebula-project-plugin.git</nebula.Module-Origin>
    <nebula.Change>0ab9baf</nebula.Change>
    <nebula.Build-Host>localhost</nebula.Build-Host>
    <nebula.Build-Job>LOCAL</nebula.Build-Job>
    <nebula.Build-Number>LOCAL</nebula.Build-Number>
    <nebula.Build-Id>LOCAL</nebula.Build-Id>
    <nebula.Created-By>1.7.0_51-b13 (Oracle Corporation)</nebula.Created-By>
    <nebula.Build-Java-Version>1.7.0_51</nebula.Build-Java-Version>
    <nebula.X-Compile-Target-JDK>1.7</nebula.X-Compile-Target-JDK>
    <nebula.X-Compile-Source-JDK>1.7</nebula.X-Compile-Source-JDK>
  </properties>
</project>
```
I think there's a lower level plugin that you could have used, since this will include CI and SCM plugins too, which might be a bit much for this specific test.