Added plugins

Hey, I noticed that the gem doesn't currently include any of the useful plugins listed at the redactor website. I've added these to their own folders in the vendor directory and created import files so that it is easy to either include all the plugins or pick and choose which plugins to include.

By default this will not include the plugins and they will only get added as the user requests these. I think this would be great to have as I tend to use the plugins often and hate having to download and include the files for them manually.

Could you have a look through my commit below and merge if appropriate, if not I'd appreciate feedback.

Thanks
.FxN