Add support for enums

[artofhuman]

Hi, it's a simple draft attempt to add support for enums (#334).
The main idea is to pass states to machine as we can do it with strings to State class and work with it.

I added some tests to illustrate the api

Example:

class States(enum.Enum):
  RED = 1
  YELLOW = 2
  GREEN = 3

m = Machine(states=States, initial=States.YELLOW)

m.states
=> OrderedDict([('RED', <State('RED')@4370833592>), ('YELLOW', <State('YELLOW')@4370472240>), ('GREEN', <State('GREEN')@4371112008>)])

m.to_RED()

m.is_RED() # => True
m.state => <States.RED: 1>

m.add_transition('switch_to_yellow', self.States.RED, self.States.YELLOW)

But now in the code base, we have many conditions isinstance(state, string_types, ) or isinstance(state, State). For support enums i added check isinstance(state, enum.Enum) and it not looking good and we should drop support for python < 3.4.

Any suggestions how enums should work are welcome :)

@aleneum what do you think about this implementation?