Interface elements for specifying raster resolution in GDAL Rasterize plugin

I've added interface elements for specifying the raster resolution when converting vector to raster data using the GDAL Rasterize plugin. So far, it has only been possible to specify the raster size.
![Screenshot of GDAL Rasterize plugin with option to specify resolution](https://f.cloud.github.com/assets/2399255/270964/89d07c8c-8fd8-11e2-99a8-64e72e356f69.png)