Oanda only provides past 180 days

The previous commit assumed Oanda only provided 180 days per request,
but they actually only provide data for the past 180 days. No data will
be returned for dates prior to 180 days ago. Update the documentation
and warning accordingly.

Also remove old code used to request the minimum amount of data from
the Oanda servers, while still fulfilling the user's request.

See #144.

R/getSymbols.R

@@ -1055,10 +1055,6 @@ function(Symbols,env,return.class='xts',
      if(!hasArg(verbose)) verbose <- FALSE
      if(!hasArg(auto.assign)) auto.assign <- TRUE
 
-     # Request minimum data from server to fulfill user's request
-     daySpans <- c(7, 30, 60, 90, 180)
-     dateStr <- c("d7", "d30", "d60", "d90", "d180")
-
      tmp <- tempfile()
      on.exit(unlink(tmp))
      for(i in 1:length(Symbols)) {
@@ -1081,15 +1077,11 @@ function(Symbols,env,return.class='xts',
        }
 
        if(verbose) cat("downloading ",Symbols.name,".....")
-       # Request minimum data from server to fulfill user's request
-       dateDiff <- difftime(to, from, units="days")
-       dateLoc <- which(daySpans >= dateDiff)
        # throw warning, but return as much data as possible
-       if(!length(dateLoc)) {
-           warning("Oanda limits data to 180 days. Symbol: ", Symbols[[i]])
-           dateLoc <- length(dateStr)
+       if(from < Sys.Date() - 180) {
+           warning("Oanda only provides historical data for the past 180 days",
+                   " Symbol: ", Symbols[[i]])
        }
-       data_range <- dateStr[dateLoc[1]]
        oanda.URL <- paste0("https://www.oanda.com/fx-for-business/",
                            "historical-rates/api/update/?&widget=1",
                            "&source=OANDA&display=absolute&adjustment=0",

man/getSymbols.oanda.Rd

@@ -40,8 +40,9 @@ available by calling \code{oanda.currencies} }
  Meant to be called internally by getSymbols only.
 
  Oanda data is 7 day daily average price data, that is Monday through Sunday.
-There is a limit of 180 days per request, and getSymbols will return as much
-data as possible and warn that the limit has been exceeded.
+Oanda only provides historical data for the past 180 days. getSymbols will
+return as much data as possible, and warn when the \code{from} date is more
+than 180 days ago.
 }
 \value{
  A call to getSymbols(Symbols,src="oanda") will load into the specified