Add keras support doc, test and evaluation example (#1012)
Add keras support doc, test and evaluation example (#1012)

* add doc, test and evaluation example

* fix bug

* comment out test

* add test back

* fix style

* fix test bug

intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL