Add bitbucket implementation of oauth2 connector.

Add interface for oauth2 connectors and a github implementation.

Update Godeps to include:

* coreso/go-oidc#24: reqiured for GitHub connector
* coreso/go-oidc#26: better error messages when connectors are misconfigured

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.

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

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