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
I can add the test, but should I be patching this in the `_.create` method or open a new PR against `_.extend`?