implement album metadata with description

The gallery generator now rejects any files in an album that do not have
a .jpg, .gif, or .png extension (the only filetypes handled by
jekyll-image-tag) and tries to read a file called "album_info.y[a]ml" to
get metadata about an album. The metadata is merged into the album
page's "data" hash and thus made available to the page's template.

Currently the gallery generator does not do anything special with this
data, but it's useful for album descriptions, etc.

Fixes #6
sort: add filename sort to album metadata

It's sort of a no-op except for the reversing, but we can now specify
"filename" as the field to sort images on.

Also add in default metadata and a bit that checks _config.yml for
site metadata defaults before applying the data from the actual
album_info.yml file.

Fixes #7