Execution environment fixes

This release fixes an important issue where specs2 execution environment is not properly shutdown.
It is strongly advised to upgrade to this version.

Note that there are some low-level breakages in this release:

• The shutdown functions on Env have changed
• A number of default methods to build low-level components like CustomInstances, SpecificationFinder, PrinterFactory have been removed. Those components now require an explicit Env
• The OwnEnv and OwnExecutionEnv traits have been reworked to operate on their own environment, without needing to make a copy of the specification Env. This avoids some easy mistakes where it is fairly easy to shutdown the specs2 execution environment
• There is a new argument, discardRejectedFutures, set to true by default, which avoids printing futures rejected by a thread pool which can happen on shutdown when testing for termination for example