Small optimization + cleaning code

[wagenaartje]

A summary of the changes I made:

• Do not use for(var i in array)! For arrays, always use for(var i = 0; i < array.length; i++). Why? Have a look yourself: https://jsperf.com/var-i-in-array-vs-var-i-is-0/1. For me, thats a 97% speed difference! So basically, I changed this in the code as much as I can.
• Instead of iterating through the list to make a 'copied' list of network.layers.hidden, and then reversing it - I simply iterate backwards over the hidden layers to propagate: https://jsperf.com/iterate-plus-reverse-vs-reverse-reverse. Thats a 83% speed difference for me.
• Using set could cause network.layers.hidden to be undefined, so instead of doing this.layers = layers, it now works like:

    this.layers = {
      input:  layers.input || null,
      hidden: layers.hidden || [],
      output: layers.output || null
    };

• --require does not come installed when you use npm install. And it is an unnecessary dependency anyways: just correctly using _inject-dist/src.js in test/ for mocha testing is far more practical. Also removed mocha.opts because it served no use.
• Added some brackets for readability (it's just personal style, so I let it pretty much unchanged mostly)
• There was difficult computation for Neuron.squash.TANH (maybe for a reason?), but I changed it to just use the built-in method Math.tanh()
• Neuron squases no longer call themselves, this is inefficient.
• network.label and node.label properties removed. They weren't used anymore and served no purpose. If a user wants to set a custom label, he/she can just do instance.label =.
• Perceptron, LSTM and Liquid all used this.trainer = new Trainer(this). Why is this not stated in the documentation? The documentation even encourages:

var myNetwork = new Architect.Perceptron(2, 2, 1)
var trainer = new Trainer(myNetwork)

So I removed the Architect constructors to automatically create a Trainer for themselves (except for Hopfield). This was only used in mocha testing anyways, which I updated.

• Actually made synaptic.min.js minified! Using uglifyJS webpack plugin. So I got these warnings: but I only changed the unused variable because I wasn't sure what the reasoning was behind the static if statements.
• Some other things

If you feel like I should change something about this, feel free to let me know! I might do some more optimization in the future.

[cazala]

Awesome @wagenaartje :D thanks for the PR, I just left one change request regarding the minification, everything else looks great 👍

[wagenaartje]

Oops, didn't know about that. I just reverted the uglifyJS in a commit and made another commit to let the TravisCI check rerun (sometimes it just fails). Should be good to go :+1: