Pome JS tests now part of build

Moved rdf/src to rdf/rdf_common_jvm/src so that IDE's work nicer. Otherwise, I have just got pome compiling OK and the tests pass.

I have also used my plugin https://github.com/InTheNow/sbt-scalajs that allows scalajs to test and compile as per a scala project - i,e

sbt test
I understand.

But here is my reasoning: `scala-js` is still young, as is `pome`/`rdf-js`. All those things are still **experimental**.

I totally understand the temptation of having them as part of the main build, but I would like to first reach a 1.0 milestone with robust typeclasses and syntaxes definitions. I honestly don't feel comfortable with having scala-js stuff being in the middle of refactoring before that.

That being said, the `reduce duplication between plantain and pome to the minimum` thing has nothing to do with having the dependencies "on" by default.

In the meantime, I think we could have a second meta project with all the JS related stuff, which could aggregate all those projects. Maybe also have a second Travis build for that, if this is possible.

As a reminder, the whole argument here is the same that we applied to the LDP store in the past, and to the LD Patch implementation more recently. Another solution would be to stop working in `master` directly, and have all those projects be maintained in separate branches, until they are deemed stable enough for `master`.

WDYT?