add keras wrapper; code refactor (#1953)
add keras wrapper; code refactor (#1953)

* TimeDistributed & Bidirectional

* Code refactor of LayerConverter
yangw1234/BigDL,yangw1234/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL,yangw1234/BigDL,intel-analytics/BigDL,intel-analytics/BigDL