Fix fatal error on Phar task (and more)

When the metadata is missing (that is possible, because its optional with phar php extension), we got a FATAL error.

    Fatal error: Call to a member function toArray() on a non-object 
    in phing\tasks\ext\phar\PharPackageTask.php on line 352

You can reproduce this issue with this phing test file 

    <?xml version="1.0" encoding="UTF-8"?>
    <project name="phar" description="Build phar archive" default="do">

        <property name="phar.basedir"    value="${env.TMP}/${phing.project.name}" />
        <property name="phar.destfile"   value="myproject.phar" />

        <target name="do">
            <mkdir dir="${phar.basedir}" />

            <pharpackage
                basedir="${phar.basedir}"
                destfile="${phar.destfile}"
            >
                <fileset dir="${phar.basedir}" />
            </pharpackage>
        </target>

    </project>    

I've also introduce some code clean-up (whitespaces), added some phpdoc descriptions missing, prevent error for PHP 5.2 users (check extension loaded)

And at least fix the fatal error.

- Laurent