Factor out plot header and footer into separate routines
Stub summarize_results_html_plot_history()
Add jqplot.dateAxisRenderer plugin for use in history plots
Add do_summary_plot() to JS, stub a summary chart, and enable history plots for testing
Minor whitespace fix
First draft of `bench --format=html_plot history`

Note: this requires a new jqplot plugin, so make sure to grab:

    jqplot/plugins/jqplot.dateAxisRenderer.min.js