dropout serialization missing

[theedge456]

I noticed that the serialisation of the type Dropout was missing on the release 0.1.0, preventing from saving and loading a trained network.
The workaround is to apply the trick described here. The problem is that it works only for ghc>=7.2.1.
Tell me if you prefer to add a fix for any ghc release or if I may pull my fix.

[HuwCampbell]

Thanks for the issue, sorry I didn't notice it earlier.

The dropout layers themselves aren't actually fully implemented, so I wasn't super worried that they weren't persisted.

I'll add a serialisation instance for them and actually try to get them going properly. The difficulty is that there's implicit randomness in a dropout layer, and I didn't want to put effects in which would pollute the purity of system.