Add support for python_platform_implementation environment marker

This patch adds support for the 'python_platform_implementation'
environment marker as defined by PEP-0426:
https://www.python.org/dev/peps/pep-0426/#environment-markers

pkg_resources/__init__.py

@@ -1403,6 +1403,7 @@ class MarkerEvaluation(object):
         'python_version': lambda: platform.python_version()[:3],
         'platform_version': platform.version,
         'platform_machine': platform.machine,
+        'platform_python_implementation': platform.python_implementation,
         'python_implementation': platform.python_implementation,
     }
 

pkg_resources/api_tests.txt

@@ -420,3 +420,6 @@ Environment Markers
 
     >>> em("python_version > '2.5'")
     True
+
+    >>> im("platform_python_implementation=='CPython'")
+    False