Fix known issues in v3.1.0

[tferr]

This addresses a couple of minor (yet annoying) issues in SNT v3.1.0:

• Issue #18
• Wrong URL when accessing Help> menu
• NPE when using SNT's API

@fiji/simple_neurite_tracer, if everybody is OK with it, I would release a new version of SNT and send it to the updater (Java8 update site).

@ctrueden, I noticed that you changed from Jenkins to Travis for building SNT artifacts... (I missed the announcement, day job is catching up with me), does it change anything when cutting a new release?

[tferr]

I will merge this now, as it seems no macros can be run in the current state.