Make ManyToManyField creation optional

I was having performance troubles with the automatic creation of ManyToManyFields by model mommy. With a huge number of tests, this could be very undesirable. So I've added a flag to Mommy class which represents the will to instanciate or not the ManyToManyFields. 

To backwards compabilities, this flag is True as default, **but** I don't know if this is a nice aproach because I think that the default should be False so model mommy wouldn't affect the tests performance very much.