Add a PURE_PYTHON env var to suppress compiling C extensions.

Test this case under both Python 2.7 and 3.3.

setup.py

@@ -41,7 +41,8 @@ Cwrapper = Feature(
 # PyPy won't build the extension.
 py_impl = getattr(platform, 'python_implementation', lambda: None)
 is_pypy = py_impl() == 'PyPy'
-if is_pypy:
+is_pure = os.environ.get('PURE_PYTHON')
+if is_pypy or is_pure:
     features = {}
 else:
     features = {'Cwrapper': Cwrapper}

tox.ini

@@ -3,12 +3,24 @@ envlist =
 # Jython support pending 2.7 support, due 2012-07-15 or so.  See:
 # http://fwierzbicki.blogspot.com/2012/03/adconion-to-fund-jython-27.html
 #   py26,py27,py32,jython,pypy,coverage
-    py26,py27,py32,py33,pypy,coverage,docs
+    py26,py27,py27-pure,py32,py33,py33-pure,pypy,coverage,docs
 
 [testenv]
 commands = 
     python setup.py test -q
 
+[testenv:py27-pure]
+basepython =
+    python2.7
+setenv =
+    PURE_PYTHON = 1
+
+[testenv:py33-pure]
+basepython =
+    python3.3
+setenv =
+    PURE_PYTHON = 1
+
 [testenv:jython]
 commands = 
    jython setup.py test -q