[pkg] Add check-manifest target in tox.ini

tox.ini

 [tox]
-envlist = py27,flake8
+envlist = py27, flake8, check-manifest
 
 [testenv]
 sitepackages = True
@@ -17,3 +17,10 @@
 [flake8]
 max-line-length = 100
 exclude = .tox
+
+[testenv:check-manifest]
+skip_install = true
+deps =
+  check-manifest
+commands =
+  {envpython} -m check_manifest {toxinidir}