Added a proportional scale method

```php
Illuminage::fit($photo, $w, $h) 
```
will resize image **proportional** to fit given bounding box. 
This **will not** upscale image, if it is smaller than bounding box.

With optional 4th param:
```php
Illuminage::fit($photo, $w, $h, true)
```
it **will** upscale the image