Remove babel polyfill

Basically, here’s what’s going on. Babel has a transformer, called
runtime, that can turn ES6 code like `new Promise()` into `new
core.Promise()`, where `core.Promise` is a polypill. But, that runtime
transformer (which I was already using), can’t polyfill some ES6 code
(e.g. `var it = "test"; it.startsWith("t");`) because it doesn't know,
e.g., that the `it` in `it.startsWith` is a string, so it doesn't know
that you're looking for `String.prototype.startsWith`. The solution to
this is to require the babel polypill, which modifies the global
`String.prototype` directly. That's what I was doing before this
commit, except that it also causes a problem: any dependents of this
library also get their globals changed.

Now, maybe that should usually be ok in practice—since most of the
polyfills match the ES6+ specs—but, of course, there's a possibility of
a polyfill having a bug, or two polyfills doing something different in
an unpolyfillable case. So, clobbering the dependent's globals is still
a bad idea. And, with this in mind, Babel actually throws an error if
the dependent tries to load babel/polyfill for a second time…so just
transparently modifying the globals and bearing the risk isn't possible
even if it's what we wanted to do.

So, this commit removes the polyfill's require, which then allows me to
move babel to a devDependency (without reopening #11, which was based
on the polyfill line). This means no using the ES6/7 `Array.prototype`
and `String.prototype` methods, but, oh well. It looks like I’d removed
all uses of them earlier anyway in preparation for this change.

Should close #47

index.js

@@ -1,5 +1,3 @@
-require('babel/polyfill');
-
 module.exports = {
   dbAdapters: {
     Mongoose: require('./build/src/db-adapters/Mongoose/MongooseAdapter')

package.json

@@ -13,7 +13,6 @@
   },
   "main": "index.js",
   "dependencies": {
-    "babel": "5.6.14",
     "babel-runtime": "5.6.17",
     "co": "4.5.x",
     "content-type": "1.x.x",
@@ -28,6 +27,7 @@
     "vary": "^1.0.0"
   },
   "devDependencies": {
+    "babel": "5.6.14",
     "babel-eslint": "3.x.x",
     "chai": "^1.9.2",
     "eslint": "0.x.x",