Add Intro to FRP post

Really interested in knowing wether this actually makes FRP easier to understand too if you don't know FRP.

And if you do know FRP please fact check me

I think revisions should work fairly well here, but we'll see.
An important note is to not make comments on the diff, but the PR otherwise when you update the pr with the changes the comment doesn't hide itself automatically (I don't think)
I think you should give a little more context. I'm also not sure if you mean with a mapping function.

Assuming mapping function:

```
// stream of keys pressed
var keys = ["p","w","n","2","o","w","n"];
// mapping function
var keyCountMapper = function(key, keyCount){
  return ++keyCount;
}
// applying the map function
var keyCount = keys.map(keyCountMapper);
console.log(keyCount); // => [1, 2, 3, 4, 5, 6, 7]
```

Assuming reducing function:
```
// stream of keys pressed
var keys = ["p","w","n","2","o","w","n"];
// reducing function
var keyCountReducer = function(reducedValue, streamSnapshot){
  return reducedValue + 1
}
// applying the reducing function
var keyCount = keys.reduce(keyCountReducer, 0);
console.log(keyCount); // => 7
```

[jsfiddle](http://jsfiddle.net/aLnL0q8L/)