add nneval, nnupdatefigures and adds plotting functionality to nntrain

nntrain changed to optonally take val_x and val_y as last to arguments.
nntrain plots training progreess if opts.plot = 1

nneval:Used for evaluating performance of network on training and
optionally validation data

nnupdatefigures: updates figures and plots the training progress.
fix comments
Added plotting function to nntrain. Removed normalizing code from nntrain.
normalization function
Merge pull request #29 from rasmusbergpalm/pr/28

Cleaned up pull request from @skaae. Thanks!