Added ext-mbstring and ext-exif to suggested / require-dev in Composer as these are not technically 100% standard.
Animation delay is now null by default, which means to keep the original timing.

Imagine/Layers now also replaces the Imagick resource in its loop, as this is actually required for any changes to frames to propagate.
Added a simple test, with an image that couldn't be resized properly before.
Small documentation update
Removed default sigma value in EffectsInterface (should not have been committed)
Amended suggestion description for ext-exif, removed exif as dependency for dev and removed mbstring entirely.
testSaveWithoutFormatShouldSaveInOriginalFormat will now be skipped if ext-exif isn't installed.
Apply an animation delay of 0 (which is perfectly valid) properly
Animated GIF resize test is now a global test (Gmagick results pending!)
Dumbass skip message.
Reverted another mistaken change in EffectsInterface
Gmagick implementation now supports inheriting image delay too
Updated AbstractImageTest to only coalesce() for Imagick
Removed ext-exif as suggestion
Merge branch 'feature/animated-gifs' of github.com:Werelds/Imagine into Werelds-feature/animated-gifs
Update changelog
Update documentation
Merge branch 'Werelds-feature/animated-gifs' into develop