ustreasuries: DF bootstrap and equity index downloads added

Seventh Development Release 0.0.0.9006

• ### Added vignette veronesi-ch02, all the examples and figures from Chapter 2 of Veronesi
• ### Added Zbootstrap function
• ### Modified FedInvestData function to start at the date following the last date in the historical file
• ### Added data-raw/FedInvestHistory_bring_to_current.R to "top off" the FedInvest historical dataset so that as few POSTS as possible have to be made to bring it current
• ### Added SP500 and SP500TR to download S&P 500 data