added the 'strip' filter

This filter comes with Imagine and reduces an image's size by removing
profiles and comments.
moved here from README.md
Merge pull request #152 from uwej711/add_strip_filter

added the 'strip' filter
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
add CacheManagerAwareTrait

The trait is given, but as the bundle is meant to work on PHP 5.3, this trait is not used by the bundle directly.
It's sole purpose is to have a common trait for applications using this bundle.
Merge pull request #173 from havvg/master

add CacheManagerAwareTrait
Allow symfony 2.3 and greater

Also run travis tests on symfony 2.2
improve phpcr loader doc
Merge pull request #177 from liip/phpcr-loader-doc

improve phpcr loader doc