makefile: Adding make stress-http LOAD=<x>

This new makefile target simulate a load of <x> simulatenous
curl requests to the upload.py script.

At the end of the run, an summary like the following is shown

 #######################
 # Stress-http Results #
 #######################
 Number of requests  :   50
 Failures counting
   CURL              :    0
   Python            :    0
   Missing files     :    7
   Total             :    7
 CURL timing
   Total Spawning  :    0  seconds
   Total Waiting   :  132  seconds
   Min waiting     :    0  seconds
   Max waiting     :   67  seconds
   Average waiting :    2,64 seconds
   Std deviation   :   11,27 seconds