Corrected BCECriterion test

Changed test so that 
a) both input and target have entries lying between 0 and 1
b) both input and target have entries at least 0.01 away from 0 / 1. This is needed to make this test stable - it otherwise fails frequently depending on random initialisation (the numerical approximation of the gradient of log(x) and log(1-x) diverges from the actual values near x=0 and x=1)