Add ability to truncate with foreign keys disabled

[josephmancuso]

Most times if you ar truncating and have foreign keys it will throw an error so need a way to disable foreign key checks when truncating

[josephmancuso]

Needs these 2 new methods on the schema class:

Schema.disable_foreign_key_constraints()
Schema.enable_foreign_key_constraints()

and possibly one method that does all of it in 1:

Schema.truncate_without_foreign_keys()

or maybe even:

Schema.truncate(foreign_keys=False)