Adding Javadoc and sources jar to build/publish

[Smithor]

Super simple addition to the build script to build the javadoc and sources. This one doesn't need to be added if you do not want it, but I am running it on my local copy so that I don't have to keep the SnapKit project open all the time.

• Add generation of Javadoc and sources jars so that they can be included at publish time.
• Fixed a Gradle duplications problem with building the sources jar.

[Smithor]

I would like this one merged, as it makes the writing of javadocs easier in IntelliJ. IntelliJ uses the source jar to copy the javadocs from, and also builds its internal javadoc reader from that source.

I honestly hardly leave the IDE, its just so convenient, but it only works fully if all the data is provided. Technically the javadoc jar isn't required for this, but having all options available is favourable, especially since it costs nothing.