Added documentation for zips module.
Removed usage of with_setup, not needed.
Fix to allow different extensions to set the same environment variable through the  method.  When two extensions return the same environment variable, it's grouped and returned in the form  where ':' is os.pathsep.
Fixed some formatting issued found with flake8.
Improved on #d845f6c.  This patch allows for extensions to specify
environment variables in two ways:  as a string or as a list.  If it's a
string, it will be added as-is.  If it's a list, the list is combined
using the path separator.  If two different extensions set the same
environment variable (for example PATH), all of the values are combined
and set into the same environment variable (dupliates are not removed).