Implementing GET '/channels/:id' & '/guilds/:id/channels'

[NCPlayz]

Summary

This adds the two known channel metadata endpoints, listed on #133.

• GET /channels/:id -> Client.fetch_channel
• GET /guilds/:id/channels -> Guild.fetch_channels

Both of these give full objects of channels, not partial objects.
You can also do Client.login -> Client.fetch_channel, or Client.login -> Client.fetch_guild -> Guild.fetch_channels.

Just a side note: weirdly enough, doing Client.fetch_channel(cannot_read_channel_id) gives Forbidden, but Guild.fetch_channels() -> utils.get id=cannot_read_channel_id doesn't.

Checklist

• If code changes were made then they have been tested.
  • I have updated the documentation to reflect the changes.
• This PR fixes an issue.
• This PR adds something new (e.g. new method or parameters).
• This PR is a breaking change (e.g. methods or parameters removed/renamed)
• This PR is not a code change (e.g. documentation, README, ...)

[Rapptz]

I don't really like using TypeError for this type of runtime error. TypeError should be reserved for "parse-time" or call-time errors that are the fault of the user. The user is not at fault here -- the library is. It needs to be a different error. Unfortunately none of the exceptions in errors.py fit this whole "unknown data received" paradigm because back then the library wouldn't care. So a new exception needs to be made for this error.