PSR-2 lint fixes in lib/ directory

Add Mac support to salt generation command

Add PHP 7.2 into Travis

Fix missing single quote in example SP Config

[FIX] ms-ds-consistencyguid need also to be Base64 encode

Fix count slave checking

The use of count function is wrong and unnecessary. If the array is
empty the foreach loop will not run.

Updated config.php template to include comma after baseURL

Fixes https://github.com/simplesamlphp/simplesamlphp/issues/818

* Check responseHeaders presence before use them

And so avoid some messages in log file.

This is minor, but if a comma is required when this is enabled the template should reflect that.

Document the new Twig templating system

It is deprecated as of PHP 7.2

Edits to CONTRIBUTING.md

This marker is used by some Markdown compilers to start the Table of
Contents hierarchy at this point.

Per suggestion from @tvdijen

Add some basic documentation on how the new Twig templating system works.

PSR-2 compliant whitespace in error classes

space is required before auth data

forward-port of my recent fix in branch 1.15