Allows _.exists _.truthy _.falsey to be variadic

`_.exists`, `_.truthy` and `_.falsey` can now be called with a variable number of arguments, for example:

```javascript
if (_.exists(req.body.username, req.body.email, req.body.password)) {
  // do something interesting
}
```

Extracted the concept of existance in a function of its own:

```javascript
function existance(arg) { return arg != null; }
```

Updated tests accordingly with a little correction of statements for `_.falsey`: it seemed that the correct statement, for ex for `_.falsey(null)`, should be `'should know that null, undefined and false are falsey'` instead of `'should know that null, undefined and false are not falsey'`.

Let me know if this is ok or if I have to change something (or if it's totally out of question :D)