BF: Fix MultiStairHandler addReponse() and next(). 

Every multi handler would lose first intensity in every staircase, which caused misalignment of intensities and
responses in the result.
e.g. 
====code start=====
from psychopy import data

startVal=0.5
stepSizes=[8,4,4]
conditions=[   {"label":"2.0","startVal":startVal,"stepType":"db","stepSizes":stepSizes,"nUp":1,"nDown":2,"minVal":0,"maxVal":1,"nTrials":12},
{"label":"4.0","startVal":startVal,"stepType":"db","stepSizes":stepSizes,"nUp":1,"nDown":2,"minVal":0,"maxVal":1,"nTrials":12}
]

stairHandler=data.MultiStairHandler(conditions=conditions,method='random')
currentStair=stair.next()
while True:
    print "cur stair:",currentStair[1]["label"],"cur contrast:",currentStair[0]
    response=raw_input("input a number:")
    stairHandler.addResponse(int(response))
    try:
        currentStair=stair.next()
    except StopIteration:
        break
stairHandler.saveAsExcel("MultiStairTest.xlsx")

=======code end========

The first intensity was 0.5, but it would lose in the excel files in all sheet. All result just misalign the intensities and responses, but reversals were not affected.   

This commit just fix the problems above.