show splash screen at startup for immediate user feedback

As of version 0.1.6 Nightcode takes several seconds to appear on the screen after it is executed. The main reason of the delay is not JVM startup time, but rather loading and initialization of Clojure and Nightcode namespaces. This PR adds a splash image to the JAR manifest (for immediate user feedback) and switches the application entry point to a Java class `nightcode.Main` that loads earlier than Clojure and Nightcode namespaces.

The Java class `nightcode.Main` loads Nightcode namespaces (hence also Clojure namespaces) via reflection while the animated splash image is displayed on the screen, then switches the display to the regular GUI frame. I generated the splash image using http://preloaders.net/en/search/text - foreground color #BACACC, background color #29291B, width 256, height 32.