Mavenizing

Hi, I mavenized the libgdx-contribs projects while working on a pet project - since maven requires all library dependencies to be saved as maven artifacts.

What's changed:
 * Directory structure has been changed to accommodate maven conventions:
    * __src__ is now __src/main/java__
    * The shaders have been moved to __postprocessing/src/main/resources__
 * The __postprocessing/package-release__ script and the Eclipse project files have been updated to reflect maven-imposed changes.
 * The demo project has been moved to the root, and has been renamed to __postprocessing-demo__.

So, in essence: The eclipse project files are preserved, but it should now be easier to work with the code from other IDE:s and CLI, and the hassle of manually downloading dependencies is removed.

Also, as per the libgdx maven archetype, to run the Demo from maven:
```
$ mvn -Pdemo integration-test
```