Extended maven-nunit-plugin to support systemProperties configuration

Summary

I have extended the maven-nunit-plugin to support the use of systemProperties. Unfortunately, as I am working from an in-house copy I can't easily make a patch file for you.

Significant Changes

1) Addition of new Properties class.
2) Several small changes within the NUnitEnvironment and NUnitMojo classes.
3) Upgrading org.codehaus.plexus:plexus-utils from version 1.0.4 to version 1.1 (necessary to gain some enhancements to the plexus Commandline class)

Unrelated Changes

1) Somewhat different POM inheritance since I'm using an in-house version. If you take the changes above, everything should work out.

Motivation

By combining this change to the maven-nunit-plugin with my change to the maven-exec-plugin (See MEXEC-4) its possible to create a cross-language integration test.
My C# based NUnit test can use the .NET Process class to execute "mvn exec:java" as part of a [TestFrameworkSetUp] annotated method in an nunit test. The exec:java goal within the C# code's POM just so happens to start up a java based server. (I actually have a little C# utility class to make this easy.) The nunit test methods can then perform integration tests requiring communication with the java server process. Upon completion of the tests a [TestFrameworkTearDown] annotated method in the nunit test can shut down the server process.

Example POM Snipt

           <plugin>
                <groupId>org.apache.maven.plugins</groupId>
                <artifactId>maven-nunit-plugin</artifactId>
                <version>1.3.1.jpm</version>
                <configuration>
                    <!--<inheritSystemProperties>true</inheritSystemProperties>-->
                    <systemProperties>
                        <systemProperty>
                            <key>basedir</key>
                            <value>${basedir}</value>
                        </systemProperty>
                    </systemProperties>
                </configuration>
                <!-- configuration>
	          <configFile>src/main/resources/App.config</configFile>
	        </configuration -->
            </plugin>