Add support for exclusion constraints

[trendels]

This should fix djrobstep/migra#147

Unfortunately this required some special casing of exclusion constraints in the SQL generation functions.

Exclusion constraints don't show up in information_schema.table_constraints, so remove that table from the constraints query, and use the equivalent fields from pg_constraint directly.

Also, the "USING INDEX" form is not available when creating an exclusion constraint. The index is always created implicitly. Instead of a "CREATE INDEX" statement, emit a dummy statement ("select 1;") for indexes that belong to exclusion constraints.

[djrobstep]

Awesome, thanks so much for contributing this! Have given it a brief lookover and all looks good, can hopefully give it a more detailed review later today and then merge.

[djrobstep]

This code, with some minor mods, is now added. Thanks so much for contributing this!