Added missing err return introduced by #199

api: validate local connector existence before creating user

Add ConnectorConfigRepo to UserManager. When trying to create a
RemoteIdentity, validate that the connector ID exists.

Fixes #198

Tweak dev guide & update example README

*: use example.com in place of coreos.com

This commit moves the user.Manage to its own package (user/manager)
so it can import the connector package in a later commit.

For clarity, it renames "Manager" to "UserManager" using gorname.

This commit has no functional changes.

readme: added godoc link to readme

Align with RFC2606 for example email addresses, using example.com
in place of coreos.com where appropriate.

- Be more explicit with exact instructions to startup
- Change some of the commands that were using local directory with "./" when
    the executable was in a lower directory. These
    commands (like "./bin/dex-worker") did not work on ZSH or Bash, so I
    removed the leading "./"
- Add some syntax highlighting markdown to code sections

documentation: clarify db permissions in getting started doc

In the 'getting started' doc create a user and database for dex to
clarify that the user needs permissions to create databases, but
dex does not.

closes #127

*: trim newlines from base64 command for Linux compatibility

base64 wraps lines on Linux after 76 characters. Use tr to trim
the newlines in a portable way.

closes #114

registration: trim spaces and sanity check user email from form

user: fix bug in ValidEmail helper

When a user attempts to register an email, trim prefixed and
trailing spaces, then perform a basic sanity check to ensure it's
of form "test@example.com".

Fixes #163

We aren't coming up if you google OAuth 2 on Google. Fix this.

mail.ParseAddress will stop parsing a string once it finds a valid
email address. This means you could give ValidUser an email
address followed by junk and it would mark it valid.

This commit fixes this behavior and adds some more test cases.

Fixes #189

New invitation endpoint

An invitation allows users to both verify their email address and set
a new password.

server: add scope to newLoginURLFromSession

need to make sure the scope is present otherwise will be considered an invalid request

fixes #135

Update README.md

fix whitespace issue

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