connector: fix path that connectors listen on

When Dex uses a non-root issuer URL, it current assumes that all
path prefixes will be trimmed by an upstream proxy (e.g. nginx).
This means that all paths rendered in HTML will be absolute to the
prefix, but the handlers still listen at the root.

Connectors are currently the only component that registers at a
non-root URL. Make this conform with the rest of Dex by having the
server determine the path the connector listens as rather than the
connector itself.

backend: add absolute path to template links

fixes: #502

Add a new template function called `absPath` that converts relative
paths to absolute paths based on the issuerURL. The template function
can be used in templates like:
````html
<a href="{{ .MyRelativePath | absPath }}">
````

cmd: update flag defaults for dev mode

Updates flags in dex-worker for no-db mode to work with default static
files.

*: depricate --email-from flag and move to email config files

Add groups scope and LDAP implementation

connector: remove erroneous logging statement

Use "plain" auth if and only if username and password are provided.

Parse port from "host" field if it's provided.

docs: fix various typos & Markdown lint issues

Documentation: clean up release documentation

Documentation: minor fix

return 409 for duplicate client ids

Clean up LDAP Connector

* Remove some unlikely to be used fields to help configurability.
  * Combined "serverHost" and "serverPort" into "host"
  * Remove "timeout" (just default to 30 seconds).
  * Remove "maxIdleConn" will add it back if users feel the need
    to control the number of cached connections.
  * Remove "trustedEmailProvider" (just always trust).
  * Remove "skipCertVerification" you can't make this connector
    ingore TLS errors.
* Fix configs that don't search before bind (previously broken).
* Add more examples to Documentation
* Refactor LDAPPool Acquire() and Put() into a Do() function which
  always does the flow correctly.
* Added more comments and renamed some functions.
* Moved methods on LDAPIdentityProvider to the LDAPConnector

return 409 status code on duplicate email errors

Fix docker-build script

*: update go-oidc to shorten length of key IDs

*: update mailgun-go to remove dependency on deleted simplehttp pkg

build with go 1.6 and actually push to quay