Update README.md

Fix link to Imagine library
Update installation.md

Update installation instructions using composer
Merge pull request #158 from dlondero/patch-1

fix link to Imagine library
Merge pull request #159 from dlondero/patch-2

update installation instructions using composer
use composer require command
Merge pull request #160 from gimler/install_composer_require

use composer require command
CS tweak
made the phpcr loader search for the requested path with or without a file extension
Merge pull request #169 from liip/phpcr_loader_tweaks

made the phpcr loader search for the requested path with or without a file extension
add background filter
adding documentation
Merge pull request #171 from maxbeutel/pr/background-filter

add background filter