add DIEN

add multi-value feature embedding with sum pooling or average pooling

adjust blank lines

add version and reference comment in each model

update README.md

Merge remote-tracking branch 'origin/master'

add din.py in sequence rec models

redesign the wrapper module(experimental)

Update README.md

add Location-based Attention in attention.py

fix some bugs in comment in nfm.py

move the factor 0.5 into bi-interaction layer in functional.py

correct NFM model structure

add uniform dimension embedding layer for embeddings with a uniform dimension

update implemented model list in README.md