Add 'appJar' and 'bootstapNodeJar' tasks to build

The build now exposes two explicit ShadowJar tasks: one for the main
JavaFX client (`appJar`) and one for the headless bootstrap node
(`bootstrapNodeJar`).

Run as follows:

    ./gradlew appJar
    -- or --
    ./gradlew bootstrapNodeJar

The resulting executable jar for each will be found in the `build/libs`
directory.

Thanks to @johnrengleman for his help at johnrengelman/shadow#108

Resolves #265
Resolves #252