Fix DSSIM definition (#99)
Fix DSSIM definition (#99)

* Fix DSSIM definition

Fix covariance definition:
https://github.com/fchollet/keras/issues/4292#issuecomment-306976842

* second fix

* fix PEP8 error

keras-team/keras-contrib,farizrahman4u/keras-contrib,keras-team/keras-contrib,stygstra/keras-contrib,keras-team/keras-contrib