Smarter passing of OAuth app creds in query params

Prior to this change, we were _always_ passing OAuth app client_id and
client_secret in the query params if present.

To prepare the way for some new app-authenticated methods, this change
makes sure those are only passed in the query params for anonymous
requests.
Merge pull request #465 from octokit/app-creds-in-query

Smarter passing of OAuth app creds in query params
Check a token using app authentication
Client#as_app for making requests on behalf of OAuth app
Application-authenticated methods for tokens
Ensure Client#authorize_url has a client_id to work with
Document Client#as_app [skip ci]
Reset cached agent when auth info changes
Fix method example in docs [ci skip]
Paginate GET /repos/:owner/:repo/git/refs
Paginate GET /repos/:owner/:repo/issues/:number/labels
Paginate GET /repos/:owner/:repo/milestones/:id/labels
Paginate GET /orgs/:org/teams
Paginate GET /repos/:owner/:repo/pulls/:number/commits
Paginate GET /repos/:owner/:repo/pulls/:number/files
Paginate GET /repos/:owner/:repo/downloads
Paginate GET /repos/:owner/:repo/labels
Paginate GET /teams/:id/members
Paginate GET /teams/:id/repos
Merge pull request #469 from octokit/always-be-paging

Update paginated endpoints to accept pagination