reshape data based on column name : How can one reshape data based on column name 


    x1	x2	x3	x4
    1	1	5	6
    1	2	3	5
    2	1	6	1
    2	2	2	4


to 



   

    x1  1   
    x1  1
    x1  2
    x1  2
    x2  1
    .
    .
    .

I have read many posts but I could not figure this out 