Replace masked code in all recurrent layers by T.switch command

[avostryakov]

Replace masked code in all recurrent layers by T.switch command

Closes #550.

[skaae]

It looks good to me. Thanks.

[benanne]

Looks good to me as well. I'll let you do the honours :)

[f0k]

Looks good, it's cleaner and shouldn't hurt (the previous implementation allows to do some tricks with non-binary masks, but probably nobody did that).
@craffel: Was there a specific reason for not using T.switch()? Did you observe that multiplication to be faster (contrary to the cursory test in #550)?

[skaae]

I think i did the masking implementation. I don't think there was a particular reason for not using T.switch

[craffel]

I think there was some reference implementation somewhere from the Theano folks who used the multiplication trick. Great that switch is faster, thanks for this.

[f0k]

All right, merging then! Thanks again!