Skip tests instead of fail'ing if VCS <foo> command not installed

I have svn, hg and git installed, but not bzr. Any VCS tests will currently fail due to a missing command. Eg: If a user only uses git, svn, hg and bzr tests will fail. 

They should instead be skipped with an appropriate message: "skipping . 'foo' command not installed" or equivalent.

```
======================================================================
ERROR: test_get_vcs_files (tests.TestBzr)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 770, in test_get_vcs_files
    self._init_vcs()
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 756, in _init_vcs
    self.vcs._init_vcs()
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 831, in _init_vcs
    self._run('bzr', 'init')
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 724, in _run
    stderr=subprocess.STDOUT)
  File "/usr/local/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/usr/local/lib/python2.7/subprocess.py", line 1335, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

======================================================================
ERROR: test_get_vcs_files_added_but_uncommitted (tests.TestBzr)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 781, in test_get_vcs_files_added_but_uncommitted
    self._init_vcs()
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 756, in _init_vcs
    self.vcs._init_vcs()
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 831, in _init_vcs
    self._run('bzr', 'init')
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 724, in _run
    stderr=subprocess.STDOUT)
  File "/usr/local/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/usr/local/lib/python2.7/subprocess.py", line 1335, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

======================================================================
ERROR: test_get_vcs_files_in_a_subdir (tests.TestBzr)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 791, in test_get_vcs_files_in_a_subdir
    self._init_vcs()
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 756, in _init_vcs
    self.vcs._init_vcs()
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 831, in _init_vcs
    self._run('bzr', 'init')
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 724, in _run
    stderr=subprocess.STDOUT)
  File "/usr/local/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/usr/local/lib/python2.7/subprocess.py", line 1335, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

======================================================================
ERROR: test_get_vcs_files_nonascii_filenames (tests.TestBzr)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 803, in test_get_vcs_files_nonascii_filenames
    self._init_vcs()
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 756, in _init_vcs
    self.vcs._init_vcs()
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 831, in _init_vcs
    self._run('bzr', 'init')
  File "/mnt/home/user/repos/freebsd/ports/devel/py-check-manifest/work/check-manifest-0.22/tests.py", line 724, in _run
    stderr=subprocess.STDOUT)
  File "/usr/local/lib/python2.7/subprocess.py", line 710, in __init__
    errread, errwrite)
  File "/usr/local/lib/python2.7/subprocess.py", line 1335, in _execute_child
    raise child_exception
OSError: [Errno 2] No such file or directory

----------------------------------------------------------------------
Ran 87 tests in 8.683s

FAILED (errors=4)
```