Add hooks for running ruby blocks before/after a backup run.

Sometimes you need to do some tricky things to put your system into a proper state for backup. This patch adds hooks for scripting these setup and teardown tasks right in your model's config.

More specifically, it adds these DSL methods:

```ruby
Backup::Model.new(:test_backup, 'back up procedure with before/after hooks') do
  before do |model|
    # a ruby block for setup
  end

  after do |model|
    # a ruby block for teardown
  end

  # the rest of your configuraiton
end
```

These hooks will be called right after logging the 'backup started' message and right before the 'backup finished' message.

Each hook also logs its own start, finish, or error messages.

Tests included.