- Changed the scheme used to generate truncated

- Changed the scheme used to generate truncated
"auto" index names when using the "index=True"
flag on Column.   The truncation only takes
place with the auto-generated name, not one
that is user-defined (an error would be
raised instead), and the truncation scheme
itself is now based on a fragment of an md5
hash of the identifier name, so that multiple
indexes on columns with similar names still
have unique names.  [ticket:1855]