Slack-sdk

Added rate limit handling for rtm connections (thanks jayalane!)

- You can now tell the RTM client to automatically reconnect by passing `auto_reconnect=True`

- Fixes Web API JSON encoding

- Fixed an encoding issue which was encoding some Web API params incorrectly (sorry)

- Adds HTTP response headers to `api_call` responses to expose things like rate limit info
- Moves `token` into auth header rather than request params

- Adds new SlackClientError and ResponseParseError types to describe errors - thanks aoberoi!
- Fix Build Error (245) - thanks stasfilin!
- include email as user property (173) - thanks acaire!
- Add http reply into slack login and slack connection error (216) - thanks harlowja!
- Removed unused exception class (233)
- Fix rtm_send_message bug (225) - thanks kt5356!
- Allow use of custom parameters on rtm_connect() (210) - thanks kamushadenes!
- Fix link to rtm.connect docs (223) - sampart!