Describing the configuration option available for password authentication.

Might not be complete, needs check with current G Suite interfaces.
But improves on the current situation.

Minor format and exception message fixes, plus add information about the different communication channels we have today.

This should help people configure the PHP session handler and mitigate issues when sessions collide. Related to #478.

Change the extension of the documentation files from .txt to .md so that they can processed as markdown and displayed in github.

* Remove 404 link to SSP news; this is covered on simplesamlphp.org now.
* Make screenshots link relative, so docs don't trigger insecure
  content warnings.
* Compress png images.

(being respectful with occurences that might change the behaviour, i.e. default database prefixes)

This is more than just cosmetics, as the rnd.feide.no links redirect
people to the SSP 1.5 documentation which is rather outdated.

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@3110 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2209 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@2206 44740490-163a-0410-bde0-09ae8108e29a

git-svn-id: https://simplesamlphp.googlecode.com/svn/trunk@1297 44740490-163a-0410-bde0-09ae8108e29a