The support for Android "dp" unit

When the user specify "dp" unit, e.g. "48x48dp ic_launcher.png", create different images for different densities in corresponding "drawable-" directories.

![generated_dirs_screenshot](https://f.cloud.github.com/assets/1065299/1157905/a7514db6-1fa9-11e3-9a0b-c65abd0d2657.png)