(cherry picked from commit f75e7670)

Apparently, links to modules must have a "./" prefix.

Closes #1103

* Cleaned up .gitignore a little bit

* Added a .gitignore to modules, ignoring all non-tracked modules.

* Fix internal markdown links in the install docs

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

* Add an extra apache access control directive
  to make it work with newer Apaches
  Source: http://stackoverflow.com/questions/23337446/getting-a-403-forbidden-error-for-simplesaml-after-apache-upgrade

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

Update the documentation. Trailing slashes are no longer required for 'baseurlpath'. This resolves #363.

* 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.

SimpleSAMLphp no longer requires mcrypt, it will only be needed depending on the configuration, so mark it as optional.