Create charleston-1859-mid.xml

Charleston 1859 Prices From "The Structure and Profitability of the Antebellum Rice Industry 1859" by Dale Evans Swan, Using Midpoint Prices in Appendix I.
Update charleston-1859-mid.xml

Added several prices (oats, corn, peas)
Entered Values for 1974 Fogel Diet Survey

Found in TIme on the Cross: Evidence and Methods- A Supplement on page 97 (Table B.13)
Created Sutch Diet

Found in: Without Consent or Contract the Rise and Fall of American Slavery; Evidence and Methods. P 293 (Table 43.1)
Merge pull request #3 from agcopland/master

Adding Historical Price and Diet Data