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

[internaut]

I added some more features to the galleries plugin. They are based on luckyalvy's fork 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 instead of RMagick which eats up less memory
• generate gallery images in a list (<ul> tag) when columns setting is 0

[landesfeind]

This is a very nice enhancement and I recommend adding it to the master branch. However, I would also add the following patch which prevents the re-usage of the generated "-thumb" files when generating galleries from sub-folders

https://gist.github.com/landesfeind/f6c949f4c14fe17d5fc3