Add possibility to add header elements to any request

[ChrisArnault]

Hi,

I have improved slumber for my own needs.
I think the change is backward compatible:

For any request you hav now the possibility to specify a new argument:

class Resource(ResourceAttributesMixin, object):
...
def (..., headers=, **kwargs):
...

Thanks for checking this modification
(of course this is running in my application)

Regards
Christian Arnault

[coveralls]

Coverage decreased (-1.1%) to 86.099% when pulling 27c8a71 on ChristianArnault:master into 4c4b7bc on samgiles:master.