Update plugin to be compatible with WP 3.9-alpha

- Replace all events to use new TinyMCE 4.* <target>.on/off(<event>)
  methods instead of <target>.<event>.add/remove.

- Clean up variable accessors. Remove duplicate var declarations.
Update version number
Update README
Update README with note for manual installs
Make javascript and style links relative
Merge branch 'timmmmyboy-WP-3.9-alpha' into WP-3.9-alpha
Update .editorconfig to match WP coding standards
Update custom tinymce theme styles for toolbar
Update tinymce plugin for WP 3.9+ compatibility
Cleanup doc strings. Remove _ fn preface
Rename mcore-image-buttons to mcore-image-toolbar
Remove custom edit/del button. Use dashicons from wp3.9
Fix incorrect param keys
Update doc str
Use tabs for indentation as per WP coding style
Add settings error styles
Update version to 2.6
[changelog] Update plugin for WP3.9 compatibility
Standardize bracket spacing
Merge branch 'WP-3.9-compatibility'