Add caching for all SBT tasks

[rethab]

• not having caching meant every time we run compile, the output files
  are deleted and freshly written. in a project with multiple modules
  and big swagger files, this leads to significant unnecessary overhead.

• also added logging so that we see when things are compiled and made
  various other cleanups in the plugin.

[rethab]

@andreaTP @fralken What do you think?

Please note that I created this PR (originally) based on the tag from 0.0.11, because that is the version we are using while master is already quite different. In fact, since master is currently not working for us and we'd have to do quite some migration (eg. see #74), how about the following:

• Keep this on 0.0.11
• For the refactoring on master, create version 0.1.0.

After that, I'd be happy to forward-port the changes, but I think the refactoring on master changes so many things that incrementing the second number should be easily justified.

[fralken]

Hi @rethab, thanks for your PR. If you need this on top of version 0.0.11 we can merge it and publish a new version, say 0.0.12. Then it would be nice for you to port this on current master.

For sure master will be version 0.1.0 because there are a lot of changes since 0.0.11 such as code generation with scalameta, no more generation of non-scala code (e.g. routes), etc.

I would like to better isolate code generation from the plugin itself, now it is tightly coupled with Play code generation. This is my roadmap towards 0.1.0. But this requires some more work.

As for issue #74, I guess this should be fixed by the latest commits on master, let me know if this is the case.

[rethab]

@fralken sounds like a plan! Your code generation things are already on master though, aren't they? If so, are you going to drop them again or how shall we do this?

[fralken]

We can create a branch from commit tagged 0.0.11, apply your PR, update version and tag to 0.0.12, publish it and then merge it to master.

[rethab]

Nice. Could you please do that?

[fralken]

Ok, I just published version 0.0.12. I created a branch cache with these commits. It would be nice if you could port this to master with a new PR.

[rethab]

See PR #76