Add an ability to migrate to applyed time

Hi,
I made some changes)
It's can be used in CI, when you need to revert migrations to some time.

You can use:
```
yiic migrate to "2014-01-01 00:00:00"
```
Script find a migration applyed before "2014-01-01 00:00:00" and revert to it

You can use any date/time syntax wich be convertet via strtotime().
Also you can use int(11)
```
yiic migrate to 1292265716
```