Add all Yahoo Finance fields to getOptionChain

Add the following fields, which completes the set of all data
currently returned.

  contractSymbol
  currency
  percentChange
  contractSize
  expiration

Convert expiration date to POSIXct to assist estimating theta decay.

Fixes #318.

R/getOptionChain.R

@@ -21,9 +21,14 @@ getOptionChain.yahoo <- function(Symbols, Exp, ...)
     # clean up colnames, in case there's weirdness in the JSON
     names(x) <- tolower(gsub("[[:space:]]", "", names(x)))
     # set cleaned up colnames to current output colnames
-    d <- with(x, data.frame(Strike=strike, 
+    d <- with(x, data.frame(ContractID= if("contractsymbol" %in% names(x)) {contractsymbol} else {NA},
+                            ConractSize= if("contractsize" %in% names(x)) {contractsize} else {NA},
+                            Currency= if("currency" %in% names(x)) {currency} else {NA},
+                            Expiration= as.POSIXct(expiration,origin="1970-01-01", tz="UTC"),
+                            Strike=strike, 
                             Last=lastprice, 
                             Chg=change,
+                            ChgPct= if("percentchange" %in% names(x)) {percentchange} else {NA},
                             Bid= if("bid" %in% names(x)) {bid} else {NA}, 
                             Ask= if("ask" %in% names(x)) {ask} else {NA},    
                             Vol= if("volume" %in% names(x)) {volume} else {NA},