Update cost.py
Update cost.py

fix some typo, including:

line 25: tf.cost.cross_entropy -> tl.cost.cross_entropy

line 44: `x = ` -> `x = output`, `z = targets` -> `z = target`, logistic loss -> binary cross entropy loss

line 81: mse = tf.reduce_sum(tf.squared_difference(output, target), reduction_indices = 1) -> mse = tf.reduce_mean(tf.reduce_sum(tf.squared_difference(output, target), reduction_indices = 1)) 
(# The original one is 'square error'; using variable name 'mse' was not so proper :D)

line 227: that apply L1 regularization -> that apply Li regularization
(# In fact, there is L2 regularization.)
zsdonghao/tensorlayer,zsdonghao/tensorlayer