enable tox for testing in virtual environments

* Next step will be supporting newer Python versions

tox.ini

@@ -0,0 +1,18 @@
+[tox]
+envlist = clean,py27,py3,py36,pypy,pypy3
+skip_missing_interpreters = True
+
+[testenv]
+deps = -Ur{toxinidir}/requirements.testing.txt
+commands = python -m nose
+whitelist_externals = pyenv install -s 2.7.11
+	                  pyenv install -s 3.6.1
+	                  pyenv install -s pypy-5.3.1
+	                  pyenv local 2.7.11 3.6.1 pypy-5.3.1
+
+[testenv:clean]
+deps = coverage
+commands = coverage erase
+
+[testenv:report]
+commands = nosetests --with-coverage --cover-package=pyley