add a synchronous implementation

for some reason, with all of the `cp -r` style modules out there, none of them have a sync version.  i've added a simple synchronous implementation which allows for:

```js
var cpr = require('cpr');

cpr.sync('/some/existing/dir', '/this/doesnt/exist/yet')
```

please note: i have not added support for any of your "options".  just a synchronous `cp -r`.