Added new features: Galleries from folders, optional mini_magick usage, etc.

I added some more features to the galleries plugin. They are based on [luckyalvy's fork] (https://github.com/luckyalvy/JekyllGalleryTag) which already fixed some bugs of the original version.

My additional features include:

* generate galleries by pointing to folders of images instead of pointing to each single file (see "Usage" in the README file)
* optionally use [minimagick](https://github.com/minimagick/minimagick) instead of [RMagick](https://github.com/rmagick/rmagick) which eats up less memory
* generate gallery images in a list (`<ul>` tag) when `columns` setting is `0`