Improve docs for the getting started guide for oauth2_proxy

Make the docs feel more like a tutorial and add some more context.

oauth2_proxy is a proxy you can put in front of any http application to
add OAUTH 2.0 auth. I have added OIDC support to this proxy and to make
it easy to get started lets add it to the fixture.

Documentation: fix link

Fix typo in README

Documentation: document connectors format.

Documentation: turn on registration in examples

fixes #169

Invite emails

Issuer name can be used in Emailer

This is so we can populate emails with things like the issuer name.

Invite emails are essentially just reset password emails with a
different template (though this can and probably will change (slightly)
in the near future)

use GCM instead of CBC

Dockerfile: use go 1.4 to build

Dockerfile: use right env var for email templates

Minor, non-content, formatting fixes. LGTM.

build: add install instructions for go-bindata

Markdown needs a newline between a paragraph and a code block for most
renderers. Fix this.

some of these items were space indented, others were tabbed. Use tabs.

Tell the user how to fix ther error! That is the best type of error
message. :)

server: Share token code

This patch proposes behavioral changes. In particular, referring
systems will need to provide client ids under all circumstances.

integration: check when there's no secret provided