Make optional struct fields pointers

This allows encoding/xml to properly comparison against nil to see if
they should be marshalled if the 'xml' tag contains ",omitempty" and for
users to test against nil to see if a field was present in the parsed
OFX.

This commit also fixes up cmd/ofx to use the new pointers.

cmd/ofx/ccdownload.go

@@ -6,7 +6,6 @@ import (
 	"github.com/aclindsa/ofxgo"
 	"io"
 	"os"
-	"time"
 )
 
 var ccDownloadCommand = Command{
@@ -48,8 +47,6 @@ func ccDownload() {
 		CCAcctFrom: ofxgo.CCAcct{
 			AcctId: ofxgo.String(acctId),
 		},
-		DtStart: ofxgo.Date(time.Now().AddDate(-1, 0, 0)),
-		DtEnd:   ofxgo.Date(time.Now()),
 		Include: true,
 	}
 	query.CreditCards = append(query.CreditCards, &statementRequest)