Don't run unnecessary tasks by default

[nwbirnie]

The gradle build runs a few unnecessary tasks by default:

• javadoc
• shadowjar

These take a long time so should only be run on request.

[jradcliff]

Perhaps already on your mind, but in addition, publishToMavenLocal probably shouldn't publish the shadowJar. We call out publishToMavenLocal in our guide and I think most users won't need the shadow JAR, so we don't want them to pay the cost for those tasks by default.

[nwbirnie]

Fixed in
#474
#475