Fix bugs and enhance usability in image-plugin

Changes:
* Keep Aspect Ratio now works from the start if activated
* Keyboard-inputs for height/width are no longer auto-corrected when
  below minimum to avoid weird changes while still typing
* Toggling aspect ratio in crop-mode no longer breaks the plugin
* Field values are correctly updated after cropping and cancelling crop
* Gradually increasing size now works while keep aspect ratio is activated
* Several documentation fixes and minor refactoring
Fix image-demo by adding necessary paths
Fix bugs when resizing while keeping aspect-ratio

* Fixed problems when resizing and releasing mouse inside image
* Repaired toggle-button functionality
Merge branch 'hotfix' of git://github.com/maschonber/Aloha-Editor into release