Including System Metric elements

Added documentation for systemmetric_item, fixed system_metric_state
typo. Fixes #14
Updating EntityItemSystemMetricIndexType documentation

Variable references and pattern matches do not appear in Items as
indicated.
Merge pull request #195 from drothenberg/systemmetric

Including System Metric Test. Closes #14.
moving annotation documentation so it will get picked up by the xsl that generates the html documentation.