Fix sys.path manipulation in pavement.py, with detailed comment

Restore reverted changes from paver part 1

pavement.py

@@ -0,0 +1,14 @@
+import sys
+import os
+
+# Ensure that we can import pavelib, and that our copy of pavelib
+# takes precedence over anything else installed in the virtualenv.
+# In local dev, we usually don't need to do this, because Python
+# automatically puts the current working directory on the system path.
+# In Jenkins, however, we have multiple copies of the edx-platform repo,
+# each of which run "pip install -e ." (as part of requirements/edx/local.txt)
+# Until we re-run pip install, the other copies of edx-platform could
+# take precedence, leading to some very strange results.
+sys.path.insert(1, os.path.dirname(__file__))
+
+from pavelib import *