Add support for migration dry runs.

[endofunky]

In dry runs, only the execution order will be logged. No migrations will be executed or marked as executed. Unfortunately, Rake already uses --dry-run, so this is added as a separate task named "evolver:migrate:dry".

Let me know if you think this is useful. At work we ran into a few cases where we needed it (after a bunch of late, complex merges).