use path.resolve instead of path.join with prefix

Now you can set base directory for temp files without modifying exports.dir on per call basis:
```js
var tmp = temp.path({
  prefix: '/mydir'
});
console.log(tmp);
```
Output:
> /mydir/11318-1165-cahfb7

This is backward compatible (just don't use leading slash in prefix:
```js
var tmp = temp.path({
  prefix: 'myfile'
});
console.log(tmp);
```
Output:
> /tmp/myfile11318-1165-cahfb7