[doc] Set release date of 3.26

[web] Fix UnboundLocalError about "exc" in ajax controller Introduced in 159dce89a145.

Link to "3.26" docs in README

Added tag 3.26.0, debian/3.26.0-1, centos/3.26.0-1 for changeset 199851fcddd4

[pkg] Version 3.26.0

[pkg] Version 3.26.0rc0

[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.

[pkg] Drop SandboxViolation verification cracking Dunno what this is for. Let's see if this work without.

[pkg] Drop custom "install_data" command Not sure what this is for, the comment mentions "egg" directory, but this is quite likely an old hack that should be unnecessary when using a recent setuptools.

[pkg] Replace custom "scripts" handling by a setuptools entry points http://setuptools.readthedocs.io/en/latest/setuptools.html#automatic-script-creation

Set a default value for arguments of cubicweb.cwctl:run() This will help making a console script entry point from this to get rid of the custom installation of scripts in setup.py. Accordingly, update all calls of this function.