Adds THUMBNAIL_KEEP_NAME which if set puts the thumbnail in a folder named
like the key in THUMBNAIL_SIZES.
Merge pull request #145 from smartass101/master

i18n_subsites: consider templates lang, expand docs, several fixes
Merge pull request #118 from deepy/master

Add option to keep original filename and put thumbnail in a folder instead
i18n_subsites: add lang_subsites template var, improve docs

- the new variable makes it easier to implement language buttons
- a short howto on language buttons is also provided
- the dictionary mapping template vars are now OrderedDict
  for consistent-looking language buttons