replaceAll treats key as regex

`replaceAll` should not treat its second parameter as a regex.

``` js
"1$a2".replace('$a', 'b'); // => "1b2" as expected
```

but

``` js
s("1$a2").replaceAll('$a', 'b').value(); // => "1$a2" but should be "1b2"
```

See also [a stackoverflow discussion](http://stackoverflow.com/questions/1144783/replacing-all-occurrences-of-a-string-in-javascript) which suggests using the regex escape function presented [here](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions#Using_Special_Characters).

It might be nice to add the regex escape function to this library too.