Resolve get_height() error when initialized using url

When initializing TimberImage with an image URL, I was receiving the following error message upon trying to access the `height` property:

```
PHP Warning:  getimagesize() [<a href='function.getimagesize'>function.getimagesize</a>]: Filename cannot be empty in *omitted*\wp-content\plugins\timber-library\functions\timber-image.php on line 32
```

The height property appeared to be working fine when initialized using an image ID, but would error if calling `get_height()` directly.

After investigating, it appears the `get_dimensions()` method is calling a non-existent property `fileloc`. This set of changes will call `getimagesize()` with the correct `file_loc` property, and set the `file_loc` property correctly in both the case of initialization through ID and through URL.