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!
i guess it's polite to null-check it, but usually we just let it fail and then blame the user. :smiley: 
e.g. `prop` does not null-check. Anyone else have an opinion on this?