Add and export xts S4 class

A few packages need xts as a S4 class. I know quantmod and spacetime
both create an xts S4 class. The class definition belongs in the
package that owns the class.

See joshuaulrich/quantmod#269

NAMESPACE

@@ -196,6 +196,9 @@ S3method(make.index.unique, POSIXct)
 S3method(is.time.unique, xts)
 S3method(is.time.unique, zoo)
 
+# export xts S4 class (for quantmod, spacetime, etc.)
+exportClasses(xts)
+
 # xts methods
 importFrom(zoo,coredata)
 importFrom(zoo, index)

R/xts.R

@@ -31,6 +31,9 @@
 #  xts methods (which match foreign conversion methods in other files)
 #  are also defined below
 
+# For S4 extensions
+setOldClass("xts")
+
 `xts` <-
 function(x=NULL,
          order.by=index(x),