Benchmarking capabilities for KAF documents.

This adds the ability to benchmark blocks of Ruby code and write the resulting
metrics to a KAF document. These metrics are stored as following:

    <KAF>
        <benchmarks>
            <benchmark name="opener-property-tagger">
                <memory_start>1238908213</memory_start>
                <time_real>5.8123</time_real>
            </benchmark>
        </benchmarks>
    </KAF>

Metrics for the following are currently present:

* Starting memory usage
* End memory usage
* Memory increase (if any)
* CPU system time
* CPU user time
* Real time
* Total time