* Check Google group membership with hasMember and get.

This PR is an enhancement built on
https://github.com/pusher/oauth2_proxy/pull/160. That PR reduces the
number of calls to the Google Admin API and simplifies the code by
using the hasMember method. It also supports checking membership in
nested groups.

However, the above message doesn't handle members who are not a part
of the domain. The hasMember API returns a 400 for that case. As a
fallback, when the API returns a 400, this change will try using the
`get` API which works as expected for members who aren't a part of the
domain. Supporting members who belong to the Google group but aren't
part of the domain is a requested feature from
https://github.com/pusher/oauth2_proxy/issues/95.

https://developers.google.com/admin-sdk/directory/v1/reference/members/get

Note that nested members who are not a part of the domain will not be
correctly detected with this change.

* Update CHANGELOG.

* Fix incorrect JSON and stop escaping strings.

* Add comments for each scenario.

[DOCS] Fix a bunch of places where the repo link was incorrect

Remove dep from Travis CI & pre-install modules

Was missed from previous switch to go modules

Fix CODEOWNERS file

Add MAINTAINERS and update CODEOWNERS

Add silence-ping-logging flag

Co-Authored-By: Joel Speed <Joel.speed@hotmail.co.uk>

Switch from dep to go modules

Correct TLS Flags broken in #186

- `logger.go` convert slice of paths to map for quicker lookup
- `options.go` combines csv paths and pingpath into slice

- Add `ping-path` option to enable switching on and passing to `logger.go`
  Default remains unchanged at: `"/ping"`
- Add note in configuration.md about silence flag taking precedence

Potential tests:
- `options.go` sets `logger.SetExcludePath` based on silence flag?
- Changing `PingPath` reflected in router?

Can probably slim down the `ExcludePath` tests.

Useful for excluding /ping endpoint to reduce log volume.
This is somewhat more verbose than a simple bool to disable logging of
the `/ping` endpoint.

Perhaps better to add `-silence-ping-logging` bool flag to `options.go` and
pass in the `/ping` endpoint as part of `logger` declaration in `options.go`.

Could be extended into a slice of paths similar to go-gin's `SkipPaths`:
https://github.com/gin-gonic/gin/blob/master/logger.go#L46

Add ability to silence logging of requests to /ping endpoint, reducing
log clutter

Pros:
- Don't have to change all handlers to set/not set silent ping logging
- Don't have to duplicate `loggingHandler` (this could be preferable yet)

Cons:
- Leaking oauth2proxy logic into `package logger`
- Defining default pingPath in two locations

Alternative:
- Add generic exclude path to `logger.go` and pass in `/ping`.

* master:
  Move docker dep commands to earlier in the build

Move docker dep commands to earlier in the build

This will let Docker cache the results of the vendor dependencies.
Making re-builds during testing faster.

Also clean-up spurious test & rm in ./configure

Make configuration consistent

Move root packages to pkg folder

Update to Alpine 3.10