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>
```
You should be calling this with a project.plugins.withPlugin(InfoBrokerPlugin) call, and I probably would have put this as the outer block.