make setting the swf path cleaner?

if we changed the return value on setting the `swfPath` to `Client` from `path`, it would be clean to require and set the path at the same time:

```js
var Clip = require('clipboard-dom').swf('/swf/ZeroClipboard.swf');
```

i'm even tempted to make it simpler:

```js
var Clip = require('clipboard-dom')('/swf/ZeroClipboard.swf');
```

but wasn't sure if there was a reason for the `!instanceof` check in the constructor.