Add label parameter to get_all_spaces method

[frankiedrake]

There's a possibility to get all spaces by a specified label in the Confluence Rest API, but it's impossible to do this via SDK right now. As a workaround I use the following:

confluence.get('rest/api/space', params={'label':'service', 'start':0, 'limit':500})

but would be nice to have it directly in get_all_spaces method, or, maybe even better, to have some **kwargs that will be passed as a uri query.
I don't have full understanding on what these labels are (b/c I have a Confluence admin who manages it), but I can see them this way (Categories section)

[frankiedrake]

I can make a PR if you whish :)

[gonchik]

@frankiedrake sure! :)