fix #28 - add command-line switch to use any skin and color theme

This PR fixes #28 non-intrusively, wherein a command-line switch can override the skin and theme resource used by Nightcode. I have added a `light.xml` resource by XORing all color values with 0xFFFFFF. Also, it automatically chooses the right theme resource as per the skin name. Build _uberjar_ and execute with `-h` switch to see the command-line options.

A screenshot with the common light theme is here: http://imagebin.org/277367

_Note:_ Specifying the theme resource is hard for the user due to Java's handling of executable JAR files. User must construct the classpath and invoke `nightcode.core` in order to specify the theme resource.