Allow fast tests to execute during standard build

Use TestNG's test groups feature to distinguish between fast, self
contained unit tests and slow integration tests that access external
services.
Move the latter ones to the integration test phase and disable them by
default.
Run all tests by default, integration tests can be disabled.

This changes the build to a more standard Maven way.
Merge pull request #79 from albers/integration-tests

Move slow tests to integration test phase, enable tests by default