Informative message if no id.vars

Sometimes there is a need to melt data "as is", i.e. with no arguments provided:
```r
df <- data.frame(a=1:3, b=4:6)
melt(df)
```
The output is 
```r
Using  as id variables
  variable value
1        a     1
2        a     2
3        a     3
4        b     4
5        b     5
6        b     6
```
The message is not very informative and may be misleading. A small "if" handles this case, changing the message.