The feature that keeps on giving, index/unique constraint autogenerate

The feature that keeps on giving, index/unique constraint autogenerate
detection, has even more fixes, this time to accommodate database dialects
that both don't yet report on unique constraints, but the backend
does report unique constraints as indexes.   The logic
Alembic uses to distinguish between "this is an index!" vs.
"this is a unique constraint that is also reported as an index!" has now
been further enhanced to not produce unwanted migrations when the dialect
is observed to not yet implement get_unique_constraints() (e.g. mssql).
Note that such a backend will no longer report index drops for unique
indexes, as these cannot be distinguished from an unreported unique
index.  fixes #185