Thread support + user api token based client

Adds support to post to threads with `message_id` or `thread_id` to push api chat messages.

Adds support for posting to chat using a user api key
```
client = Flowdock::Client.new(api_token: '<user api token>')
client.chat_message(flow: '<flow_id>', content: 'Messaging!') # post a chat message
client.chat_message(flow: '<flow_id>', content: 'Commenting!', message: 1234) # post a comment message
```
Could we even go as abstract as having `post` and `get` to interact with the REST API? That way you could theoretically use any endpoint we have in the REST API. Obviously `handle_response` would have to return the response JSON with `get` at least.

Convenience methods like the above `chat_message` are good for the common cases. :+1: