nginx auth_request: fix -skip-provider-button

Updates README.md with svg badge

strip all tokens

gracefully report un-parsed upstream URL

Remove check for >0 upstreams

Update cookie generation to match base64 encoding

upstreamURL is a nil pointer if there is an error parsing --upstream

When used solely for auth_request there is no upstream.
Instead of forcing users to set a dummy upstream, remove
the check.

Current code is using URLEncoding but example was using the
standard RFC 4648 encoding. Switch to using the URL
encoding in the example as well.

Update Google Auth Provider instructions

Fix spelling mistake in docs

[github provider] use Authorization header, not access_token query parameter

README: nginx auth_request example updates

how to pass back the refreshed oauth2_proxy cookie from an nginx auth_request

/oauth2/auth is not more sensitive than other /oauth2/ paths,
does not need "internal" protection

"spdy" protocol is obsolete, http2 is the thing to enable now.
But it's orthogonal anyway.

No need for two separate content/upstream location blocks in
this example, reduce to just one, with a comment that it could
be serving files instead of proxying.

dist.sh: use go build option to strip binaries

30% release binary size reduction

Release v2.2

Fix url parse error

#369: Optionally allow skipping authentication for preflight requests

various fixes for getting Nginx auth_request mode working

This is useful in Nginx auth_request mode, if a 401 handler is
configured to redirect to the sign-in page. As the request URL
does not reflect the actual URL, the value is taken from the
header "X-Auth-Request-Redirect" instead. Based on #247

This is enhancement of #173 to use "Auth Request" consistently in
the command-line option, configuration file and response headers.
It always sets the X-Auth-Request-User response header and if the
email is available, sets X-Auth-Request-Email as well.

Option to skip SSL verification

travis: run gofmt and go vet