Fixup JvmCompile to always deliver non-None products that were required by downstream.

This change augments GroupTask with a pre_execute complement to post_execute and
uses this lifecycle point to ensure JvmCompile delivers on its promise of supply product maps
regardless of the availability of chunks in the round.  Further adds tests for the new lifecycle
as well as coverage for pre-existing un-tested always-run semantics of construct, prepare and
post-execute.

Testing Done:
Travis CI went green here: https://travis-ci.org/pantsbuild/pants/builds/31199740

Also:
==
$ PANTS_DEV=1 ./pants goal test tests/python/pants_test/tasks/:group_task --test-pytest-options=-v
...
21:12:20 00:01   [test]
21:12:20 00:01     [pytest]
21:12:20 00:01       [run]
                     ============== test session starts ===============
                     platform linux2 -- Python 2.6.9 -- py-1.4.22 -- pytest-2.6.0 -- /usr/bin/python2.6
                     plugins: cov, timeout
                     collected 5 items

                     tests/python/pants_test/tasks/test_group_task.py@57::GroupIteratorSingleTest::test PASSED
                     tests/python/pants_test/tasks/test_group_task.py@73::GroupIteratorMultipleTest::test PASSED
                     tests/python/pants_test/tasks/test_group_task.py@179::GroupTaskTest::test_groups PASSED
                     tests/python/pants_test/tasks/test_group_task.py@232::EmptyGroupTaskTest::test_groups PASSED
                     tests/python/pants_test/tasks/test_group_task.py@263::TransitiveGroupTaskTest::test_transitive_groups PASSED

                     ============ 5 passed in 0.17 seconds ============

And tested this change live against an internal use-case that was failing before this as such:
==
...
  File "/Users/david/.pex/install/pantsbuild.pants-0.0.21-py2-none-any.whl.0ce746980853d744737145525a44d8f91b0c5d84/pantsbuild.pants-0.0.21-py2-none-any.whl/pants/engine/round_engine.py", line 42, in attempt
    task.execute()
  File "/Users/david/.pex/install/twitter.common.pants-0.4.0-py2-none-any.whl.e1f378c90f5282b48c1ebc94c79373050751b771/twitter.common.pants-0.4.0-py2-none-any.whl/twitter/common/pants/jvm/args/tasks/resource_mapper.py", line 99, in execute
    self._add_args_resources(target, resources_by_target, transitive=False)
  File "/Users/david/.pex/install/twitter.common.pants-0.4.0-py2-none-any.whl.e1f378c90f5282b48c1ebc94c79373050751b771/twitter.common.pants-0.4.0-py2-none-any.whl/twitter/common/pants/jvm/args/tasks/resource_mapper.py", line 118, in _add_args_resources
    collect_resource_dirs(target)
  File "/Users/david/.pex/install/twitter.common.pants-0.4.0-py2-none-any.whl.e1f378c90f5282b48c1ebc94c79373050751b771/twitter.common.pants-0.4.0-py2-none-any.whl/twitter/common/pants/jvm/args/tasks/resource_mapper.py", line 110, in collect_resource_dirs
    mapping = classes_by_target.get(tgt)
AttributeError: 'NoneType' object has no attribute 'get'

Notably, this is the t.c.pants args-apt plugin [1] and it requires 'classes_by_target' [2], but gets None [3] prior to this fix.
[1] https://pypi.python.org/pypi/twitter.common.pants/0.4.0
[2] https://github.com/twitter/commons/blob/master/pants-plugins/src/python/twitter/common/pants/jvm/args/tasks/resource_mapper.py#L89
[3] https://github.com/twitter/commons/blob/master/pants-plugins/src/python/twitter/common/pants/jvm/args/tasks/resource_mapper.py#L105

Bugs closed: 415

Reviewed at https://rbcommons.com/s/twitter/r/794/