Added channel and message API calls

The following calls have been added to the API. If **args is given, then other optional parameters can be included.

* getChannels(**args)
    When given an optional ids="id,id,id", returns info about multiple channels. When no ids are given, returns info about current user's subscribed channels.

* getAChannel(chan, **args)
Retrieve a channel

* createChannel(**args)
Create a channel

* getMyChannels(**args)
Retrieve my channels (all channels current user created).

* getUnreadPMChannels(**args)
Retrieve number of unread PM channels

* updateChannel(channel_id, **args)
Update a channel

* subscribeChannel(channel_id)
Subscribe to a channel

* unsubscribeChannel(channel_id)
Unsubscribe from a channel

* getChannelSubscribes(chan, **args)
Retrieve users subscribed to a channel.

* getChannelSubscribeIds(chan, **args)
Retrieve user ids subscribed to a channel.

* getMultipleChannelSubscribeIds(**args)
Retrieve users subscribed to multiple channels

* muteChannel(chan, **args)
Mute a channel

* unmuteChannel(chan)
Unmute a channel

* getMutedChannels(**args)
Get current user's muted channels

* getMessageChannel(chan, **args)
Retrieve the messages in a channel.

* createMessage(chan, **args)
Create a message.

* getMessage(chan, msg,  **args)
Retrieve a message.

* getMultiMessages(**args)
Retrieve multiple messages. ids="id,id,id" are required.

* getMyMessages(**args)
Retrieve my messages

* deleteMessage(chan, msg)
Delete a message.

In the source file, all the new calls include a link to the documentation.