Fixes #244

The RateLimit() func is deprecated, and should not appear in the
official example.

As of [fccbb9520b (diff-e754c06a3071f63f53cee61571452b4c),) `Users.List` method was updated to `Users.ListAll`. This commit updates the documentation for `UserListOptions` to reflect that change.

API URLs starting with "/" does not play well with Github Enterprise,
whose API endpoint looks like "http(s)://hostname/api/v3/".

This fixes incorrect URLs in miletones APIs.

This change adds the fields "DiffHunk", "OriginalPosition", and "OriginalCommitId" to the PullRequestComment struct. These fields provide important information about the location of a comment.

This is a copy of commit d6bce32a36 from
Go core.

Fix a typo

- GetOrgMembership updated to accept user parameter (BREAKING CHANGE)
- EditOrgMembership updated to accept user parameter (BREAKING CHANGE)
- Added new RemoveOrgMembership method

Ref #187

- Add new options struct to AddCollaborator (BREAKING CHANGE)
- Add Permissions field to User struct, which gets populated when
  calling ListCollaborators

AddTeamRepo's "permission" option and AddTeamMembership's "role" option
should both be passed as part of a JSON request body, not as URL query
parameters.

- AddTeamMembership and ListTeamMembers both take new Option structs to
  specify role.  (BREAKING CHANGE).
- Add Role to ListMembersOptions
- update Permission and Role docs

Ref #187

update CreateTeam and EditTeam to pass preview media type if
Team.Privacy is set.

Ref #187

- allow setting permission when adding a team repository
- update IsTeamRepo to return a *Repository instead of a bool, since the
  Repository contains the permission data for the team.  This is a
  BREAKING CHANGE, however I don't think this is an often used method.
- update docs for setting the team permission when creating or editing a
  team.

Ref #187