Fixes to permission calculations

[jlu5]

This patch seems to fix some permissions calculation issues, from #98 (comment)

• When I have a permission that is TRUE for the guild and NEUTRAL for the channel, channel.get_permissions() returns False for any UID and permission (even when I'm the server owner!).
• When I have a permission that is TRUE for the guild and FALSE for the channel, disco hits a TypeError when calculating permissions:

2019-02-11 22:57:18,555 [INFO] (discord) bursting guild 497939890063802369/OVD-NET
Traceback (most recent call last):
  File "src/gevent/greenlet.py", line 716, in gevent._greenlet.Greenlet.run
  File "/home/pylink/.local/lib/python3.5/site-packages/holster/emitter.py", line 72, in __call__
    return self.callback(*args, **kwargs)
  File "/home/pylink/.local/lib/python3.5/site-packages/disco_py-0.0.13rc2-py3.5.egg/disco/bot/plugin.py", line 335, in dispatch
    result = func(event, *args, **kwargs)
  File "/home/pylink/pylink-discord/protocols/discord.py", line 169, in on_server_connect
    self._burst_guild(event.guild)
  File "/home/pylink/pylink-discord/protocols/discord.py", line 73, in _burst_guild
    self._burst_new_client(guild, member, pylink_netobj)
  File "/home/pylink/pylink-discord/protocols/discord.py", line 130, in _burst_new_client
    channel_permissions = channel.get_permissions(member)
  File "/home/pylink/.local/lib/python3.5/site-packages/disco_py-0.0.13rc2-py3.5.egg/disco/types/channel.py", line 176, in get_permissions
    denies |= overwrite.deny
TypeError: unsupported operand type(s) for |=: 'int' and 'PermissionValue'
2019-02-12T06:57:18Z <Greenlet "Greenlet-0" at 0x7f8e55a55e48: <holster.emitter.EmitterSubscription object at 0x7f8e55a34240>(<disco.gateway.events.GuildCreate object at 0x7f8e)> failed with TypeError

[b1naryth1ef]

Looks fine, permissions are easily the most complicated aspect of Discord (and honestly I don't think anyone gets it 100% right) so I'd love to get a good testing framework around this in the future. That said, thanks for getting this fix in!