Add deferrable migrations

[karenc]

When a migration's up function has @deferrable, the migration is automatically set to "deferred". It will run when migrate --run-defers is called.

Close Connexions/db-migrator#2

[karenc]

Deferred Migrations

[coveralls]

Coverage increased (+0.2%) to 97.032% when pulling 554b956318cd625603c9a59d05dbef7ab0844908 on deferred-migrations into 7cfa9fb0ca271b61974bd279458313e88dbf92f4 on repeat-migrations.

[coveralls]

Coverage increased (+0.2%) to 97.029% when pulling 14fb0061b4d976a2fbe96c187e9113c57e412216 on deferred-migrations into b4839b805ff59cc803899174fefd55d50031fc51 on repeat-migrations.

[coveralls]

Coverage decreased (-0.02%) to 96.827% when pulling 1c6df6f8903a9be71880a8d47810dce3fb71de9a on deferred-migrations into b4839b805ff59cc803899174fefd55d50031fc51 on repeat-migrations.

[coveralls]

Coverage decreased (-0.02%) to 96.827% when pulling 7adcc10172f67e3d003630b665ed0b5ce6651954 on deferred-migrations into b4839b805ff59cc803899174fefd55d50031fc51 on repeat-migrations.

[coveralls]

Coverage increased (+0.1%) to 96.979% when pulling d7fe839c76016b0dbb27ca88fdb07db839f11217 on deferred-migrations into 00e4473201dc3873c779b1d7de0a3d9d736fb78e on repeat-migrations.

[coveralls]

Coverage increased (+0.1%) to 96.979% when pulling 1426edba2bf1ab8429cf38d8109b3a36e43d5cbc on deferred-migrations into 00e4473201dc3873c779b1d7de0a3d9d736fb78e on repeat-migrations.

[coveralls]

Coverage increased (+0.1%) to 96.988% when pulling f4b04eecae7e7c62aefbdbd343adf1af722f3547 on deferred-migrations into 00e4473201dc3873c779b1d7de0a3d9d736fb78e on repeat-migrations.

[reedstrm]

I don't think this needed the name change on the decorator, but it's working fine for me for everything else. Feel free to merge.

[coveralls]

Coverage increased (+0.1%) to 96.988% when pulling dbde51feb5d7ba6ee4c3d58405901a1a6b6bea74 on deferred-migrations into 00e4473201dc3873c779b1d7de0a3d9d736fb78e on repeat-migrations.