Fixing issue with import order when using in Python 3

In Python versions >= 3.1, other importers get added to the
sys.meta_path. Unfortunately, this causes installed packages
to override Pynsive loaded plugins. Causing things like
coverage information to be improperly reported on Pynsive loaded
modules in Python 3. This also resolves:
jmvrbanac/Specter#49

pynsive/plugin/manager.py

@@ -11,7 +11,13 @@ class PluginManager(object):
     """
     def __init__(self):
         self.finder = ModuleFinder()
-        sys.meta_path.append(self.finder)
+
+        # Newer Python versions add other importers which can override our
+        # modules. Making sure we're first on newer Python version.
+        if sys.version_info >= (3, 1, 0):
+            sys.meta_path.insert(0, self.finder)
+        else:
+            sys.meta_path.append(self.finder)
 
     def destroy(self):
         """

tests/manager_test.py

@@ -0,0 +1,19 @@
+import sys
+import unittest
+
+import pynsive
+
+
+class WhenCreatingThePluginManager(unittest.TestCase):
+    def setUp(self):
+        self.manager = pynsive.PluginManager()
+
+    def tearDown(self):
+        self.manager.destroy()
+
+    def test_correct_meta_path_insertion(self):
+        finder_index = sys.meta_path.index(self.manager.finder)
+        if sys.version_info >= (3, 1, 0):
+            self.assertEqual(0, finder_index)
+        else:
+            self.assertEqual(len(sys.meta_path) - 1, finder_index)