added kwargs support, when creating new model (creating with on-liner)

Eg.

	from redisca import Model
	from redisca import String

	class User (Model):
	    username = String(
	        field='username',
	        index=True,
	        unique=True,
	    )
	    real_name = String(
	        field='real_name'
	    )

	User.new(username='tanel', real_name='Tanel').save()
	assert 'Tanel' == User.username.find('tanel')[0].real_name