Docstring improvements

[mmiller-max]

Hi there, I've been enjoying using this package and am keen to help improve docstrings. My motivation for this is I've been having to check the source code to find kwargs/default values of kwargs. A lot of this may be fine for someone who is more familiar with AMQP and that may be why there is not a lot of detail in it, but do you think the below change to the queue_declare is useful? If so I can update a few more (e.g. for the functions that are mentioned in the various markdown docs).

    queue_declare(chan::MessageChannel,
                  name::String;
                  passive::Bool=false,
                  durable::Bool=false,
                  exclusive::Bool=false,
                  auto_delete::Bool=false,
                  nowait::Bool=false,
                  timeout::Int=DEFAULT_TIMEOUT,
                  arguments::Dict{String,Any}=Dict{String,Any}())

Declare a queue (or query an existing queue).

To let the broke decide on the queue name, set `name` to `""`.

Returns a tuple: (boolean success/failure, queue name, message count, consumer count)

# Optional Arguments
- `passive::Bool=false`: if `true`, the server will reply with Declare-Ok if the queue already exists with the same name, and raise an error if not. 
- `durable::Bool=false`: if `true` the queue will survive a broker restart.
- `exclusive::Bool=false`: if `true` the queue will only be used by only one connection and the queue will be deleted when that connection closes.
- `auto_delete::Bool=false`: if `true` when the queue has had at least one consumer it is deleted when last consumer unsubscribes.
- `nowait::Bool=false`: if `true` the server will not respond. Errors will be raised via connection or channel exception.
- `timeout::Int=DEFAULT_TIMEOUT`:
- `arguments::Dict{String,Any}=Dict{String,Any}()`:  used by plugins and broker-specific features such as message TTL, queue length limit, etc.

Let me know, I'm happy to go through and add more docstrings if it'll be useful.

[tanmaykm]

Sorry, noticed this perhaps a bit late. Sure, it will definitely be useful. Please feel free to put up a PR with more docstrings!