Added methods to allow decay on MLP's with pretrained layers

Added get_weight_decay and get_l1_weight_decay methods to
PretrainedLayer class.  Both methods return 0

An alternative behavior would calculate a decay for these layers, but
it would be fixed in each iteration of the MLP.
Moved handling of decay for unsupervised and pretrained layers to decay class

The pretrained layer throws a NotImplementedError exception for decay.
The decay class handles this exception by returning 0 if the decay
coefficient is 0.
Corrected handling of weight decay for classes where it is not implemented

Fixed error message so it lists the layer type that is not implemented.

Moved methods raising NotImplementedError to Layer base class.
Merge pull request #258 from dansbecker/master

Added get_decay methods to Pretrained Layer to allow use in MLP's with decay in cost