Add new syntax for conditional event transitions

What about a little syntactic sugar for conditions?
I think this more readable:
```ruby
state :off
  event :turn_on, :transition_to => :low_battery, :if => :low_battery_level?
end

def low_battery_level?
  battery.between?(0, 10)
end
```
than:
```ruby
state :off
  event :turn_on, :transition_to => :low_battery, :if => proc { |device| device.battery.between?(0, 10) }
end
```

And why you add Gemfile.lock to .gitignore? Without it difficult install compatible gems for running tests.