Add integration tests

Resolution:
The test infrastructure and a first batch of tests were introduced in alpha1.
We switched to JUnit in the meantime, the infrastructure provides JUnit rules.

Initial description:
The 4.x tests so far consist of only unit tests, which we define as tests that don't start an external process. Obviously, we'll want more coarse-grained tests, with real over-the-network interactions with either a mock server (Scassandra / Simulacron), or ultimately a real Cassandra instance.

Create a new module for the test infrastructure (contains the equivalent of 3.x's CcmBridge, CCMTestsSupport, etc.). Suggested artifact id: java-driver-test-infra. This is primarily for our internal use, but will be published to central as well for external users. For this reason I think we should commit to semver with an api/internal package scheme, like the rest of the project.

Create a new module for the integration tests. Suggested artifact id: java-driver-integration-tests. This one is not published in central, only run as part of the build. This structure avoids cyclic dependencies:

We also keep the two test suites cleanly separated, allowing distinct schedules (e.g. unit tests on each commit, integration tests nightly).

On a side note, I'd like to point out that 4.x is a blank slate that gives us the opportunity to aggressively get rid of complexity; frankly, I find the annotations in 3.x daunting. I wouldn't mind having a bit more code to write in each test in exchange for less "magic", and more understanding of what's going on (but that's up for debate).
In the same vein, I don't know how far we want to go with test framework integration. We need to find a good balance between supporting every test framework under the sun, or limiting our dependencies and API surface. Maybe external users can write their own Junit rule.