Fix masking in merge layers.

Fixed bug where base Layer class was not handling masking with input structures correctly, which affected all merge layers.
Fixed bug where the zeros outside of the mask were still used to compute the output of the merge layer (affected Maximum, Minimum and Multiply).
Fixed bug where the output mask was not computed correctly (affected all merge layers).
Fixed bug where Concatenate layer was not broadcasting the mask correctly.

keras-team / keras