Use a specified filename to store downloaded bleu script.

[wb14123]

This patch can avoid generate lots of temp files for bleu script.

Because the training will take much time and count the bleu metrics many times, there will be lots of temp files after the training.

[wb14123]

Hi, how is this PR going? I think this is really useful.

[amirj]

Why the script should download multiple-bleu.perl multiple times?

[wb14123]

I think with this we could make sure that we are using the newest version.

But in a single training, it is better to not download the script every time we need to count bleu score. However it is a little difficult to implement since we need to pass a flag to check if it is the first time to count it.