Add "testcover", "testpod" and "testpodcoverage" targets to Makefile
Makefile: update list of secondary targets

 - Missing one "Build"

 - */MANIFEST should also not disappear for each invocation of "make"
Makefile: add a "testcritic" convencience target

 - the tests are already present, but not used by default
remove POD tests already provided by Module::Build

Module::Build has functionality for this, so we do not need to repeat this in our unit tests. The
following convenience targets has been added to Makefile already:

 - "make testpod" for POD syntax checs

 - "make testpodcoverage" for POD coverage

 - "make testcover" for test (and POD) coverage.
Merge pull request #238 from ssm/feature/pod-maintenance

Feature/pod maintenance