Update README :
 - need to download composer
 - markdown format for shell commands

Fix DomCrawler version to 2.1.x because of bug in Link.php
Fix default langage to en_UK by configuration
Add favorites
Merge from piwigo-svn (revision 21963)
Piwigo based themes and plugins should not be ignored (not in .gitignore)
Use idiorm instead of redbean for database initialization
Try to add some methods to initialize things for tests