Add Channel.permissions_for and PrivateChannel.permissions_for.

These functions handle permission resolution for a specific member.

Aids with #18.

discord/permissions.py

@@ -24,6 +24,16 @@ FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 DEALINGS IN THE SOFTWARE.
 """
 
+def create_permission_masks(cls):
+    cls.NONE = cls(0)
+    cls.ALL = cls(0b00000011111100111111110000111111)
+    cls.ALL_CHANNEL = cls(0b00000011111100111111110000011001)
+    cls.GENERAL = cls(0b00000000000000000000000000111111)
+    cls.TEXT = cls(0b00000000000000111111110000000000)
+    cls.VOICE = cls(0b00000011111100000000000000000000)
+    return cls
+
+@create_permission_masks
 class Permissions(object):
     """Wraps up the Discord permission value.
 
@@ -53,6 +63,21 @@ class Permissions(object):
         else:
             raise TypeError('Value to set for Permissions must be a bool.')
 
+    def handle_overwrite(self, allow, deny):
+        # Basically this is what's happening here.
+        # We have an original bit array, e.g. 1010
+        # Then we have another bit array that is 'denied', e.g. 1111
+        # And then we have the last one which is 'allowed', e.g. 0101
+        # We want original OP denied to end up resulting in
+        # whatever is in denied to be set to 0.
+        # So 1010 OP 1111 -> 0000
+        # Then we take this value and look at the allowed values.
+        # And whatever is allowed is set to 1.
+        # So 0000 OP2 0101 -> 0101
+        # The OP is (base ^ denied) & ~denied.
+        # The OP2 is base | allowed.
+        self.value = ((self.value ^ deny) & ~deny) | allow
+
     @property
     def can_create_instant_invite(self):
         """Returns True if the user can create instant invites."""