ENH: redo tearsheet

[oronimbus]

Planning on redoing the tearsheet and updating some of the components:

• move computations into separate classes
• add risk free rate for ratios and add flexibility to excess return (relative to market)
• add average time-under-water, outperformance, Calmar/Sortino/Treynor
• add standard errors where applicable
• if benchmark is selected, add significance test a la Ledoit & Wolf (2008)
• visualisations using matplotlib