[schema] Define a decorator to handle methods override
There is a number of external classes (from yams/rql) methods overridden
"by hand" in cubicweb/schema.py. Define a single decorator to factor out
the pattern. It handles specifying the method name (to avoid conflict
with the local namespace) and passing the original method to the new
definition when needed.
[pkg] Use setuptools.find_packages()
We drop the custom get_packages() function in setup.py that used to look
for test files (not discovered automatically since test directories are
not packages) and replace it by explicit entries in MANIFEST.in.