Introduce extra branches for unreleased features per module

[croesch]

I thought about how we should handle the examples for Swing and Core in future. Could you please have a look at ba2a8ff and review my decision!?

Previously with-latest-snapshot-versions was supposed to contain
examples for unreleased features of all AssertJ modules. Since at least
Core and Swing modules are not released in parallel, it's hard to merge
that branch to master when either Core or Swing module would be
released. Either way the master would contain examples for unreleased
features for the other module.

Therefore it might be a good idea to use several branches for tracking
the examples for new features for the different modules. Currently
I adde with-latest-swing-snapshot for the Swing module.

[joel-costigliola]

I see your point and I agree it should be easier to build and run assertj-examples when you are only interested in unreleased features belonging to a specific module.

I will add branches for guava and joda-time modules and probably keep with-latest-snapshot-versions as a mix of unreleased features of different modules to check that all module integrate well together. This also means additional work to keep with-latest-snapshot-versions up to date, we'll see how it goes in the long run.