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
```