Merge branch 'release/v0.5.1'

* release/v0.5.1:
  Bumped version to v0.5.1
  Loosening version constraint on doctrine/collections
Adding ChannelsJoin Command to API service description

See https://api.slack.com/methods/channels.join for more information.
Adding ChannelsSetPurpose Command to API service description

See https://api.slack.com/methods/channels.setPurpose for more information.
Correct indentation
Merge pull request #7 from tijsverkoyen/master

Extra methods