add seqtext_print for seqToseq demo

[luotao1]

1. seqToseq能打印出生成结果如下（与v1版本的打印结果做过比对，是一样的）：

src: <s> La dispute fait rage entre les grands constructeurs aéronautiques à propos de la largeur des sièges de la classe touriste sur les vols <unk> , ouvrant la voie à une confrontation <unk> lors du salon aéronautique de Dubaï qui a lieu de mois-ci . <e> 

prob = -28.117115: The dispute is between the large aviation manufacturers about the width of the tourist seats on the <unk> flights , paving the way for a <unk> confrontation at the Dubai aviation fair , which is a month . <e>
prob = -28.716934: The dispute is between the large aviation manufacturers about the width of the tourist seats on the <unk> flights , paving the way for a <unk> confrontation at the Dubai aviation fair , which takes place months . <e>
prob = -29.341486: The dispute is between the large aviation manufacturers about the width of the tourist seats on the <unk> flights , paving the way for a <unk> confrontation at the Dubai aviation fair , which takes place in month . <e>

2. 在wmt14.py里面，增加取字典和gen数据的函数。

[reyoung]

Excellent

[jacquesqiao]

LGTM！