Added minimal Gradle support to jpf-core

[jeandersonbc]

Minimal Gradle support consists in:

1. Gradle wrapper configured;
2. Minimal working build script

Note that testing is not currently supported but it will be supported as I evolve the Gradle support.
This Pull Request closes #43

[jeandersonbc]

Curiously, TravisCI attempted to run the build with Gradle automatically. Seems like we will have a broken CI in the gradle branch for a while

[cyrille-artho]

Could you please add a target "check" that succeeds, so Travis will not report a failure?
It's OK if we don't run the unit tests for the time being, but I would like to have a passing CI check if possible, so we know that the build works as expected.
So the temporary target "check" can be empty as long as it passes. (There may be other targets that Travis expects; please check after the next commit.)

[jeandersonbc]

All good now. The Ant build is running before the dummy Gradle build.