Some more loss functions

I've tried to add some more loss functions according to the style of activations.jl
A nice summary on different loss functions and their properties (among other things) is [1] (in particular, it suggests using the hinge loss, since it guarantees a margin of at least m)

[1] A Tutorial on Energy-Based Learning - http://yann.lecun.com/exdb/publis/pdf/lecun-06.pdf