Add _.create method

`_.create` is useful as both a "polyfill" for `Object.create` and a utility for simple prototype inheritance.

Implementation is adapted from Lodash's `_.create` method. I've kept the `baseCreate` function for the usage in the methods @jdalton [mentioned here](https://github.com/jashkenas/underscore/pull/1901#issuecomment-60267844).

```js
function Person(name, occupation) {
  this.name = name;
  this.occupation = occupation;
}

function Developer(name) {
  Person.call(this, name, 'developer');
}

Developer.prototype = _.create(Person.prototype, {
  constructor: Developer
});

var developer = new Developer('Jeremy Ashkenas');
developer instanceof Developer;
// >> true

developer instanceof Person;
// >> true
```

Ref: @jdalton https://github.com/jashkenas/underscore/pull/1901#issuecomment-60267844
You should add a test for `_.create(Person.prototype, { constructor: Developer });` as well. Underscore will fail this at the moment in IE < 9 because of object iteration issues. They've tackled it in `_.keys` but because their current `_.extend` doesn't use `_.keys` it will hit the issue.