Add R.has function and tests

I put together this PR to add the `R.has` function. It is similar to [underscore's](http://underscorejs.org/#has) `_.has` function, except with the arguments reversed to make partial application more useful. Here are a couple of examples of its use:
```javascript
var obj = {
  foo: 1,
  bar: 2,
};
R.has('foo', obj);  //=> true

var list = [{foo: 1}, {foo: 2}, {bar: 3}];
R.filter(R.has('foo'), list);  //=> [{foo: 1}, {foo: 2}]
```
This is one of the few functions that I find myself reaching back to underscore for so it would be great to get an even better version of this in Ramda!
@megawac:

> I personally would be confused if `has` didn't check own properties.

Why is that?  I've never used the Underscore method, so I don't have a certain notion of how such a function should work, but to me it sounds like one that would ask the question about whether the property in question exists on the object.  In our prototypal language, that to me is equivalent to asking if any member of the object's prototype change contains that property.  Certainly a case can be made for Underscore's behavior, but it doesn't seem like the correct behavior for a function named `has`.  (Or it seems that `has` is the wrong name for the proposed function; either way.)