Add Asset Path Tag to plugin list in docs

Requested by @mattr-
https://github.com/mojombo/jekyll/issues/881#issuecomment-27276509
Plugin description updates for Monthly/Category archive generator
Updated docs to match reality, where drafts are dated based on file modification time

See https://github.com/mojombo/jekyll/blob/8e7b6bf5ff56e6bb8d0f1019c19b351f1d600e39/lib/jekyll/draft.rb#L28 -- source uses `File.mtime` on the draft file, not `Time.now` as the doc says.
Update jekyllrb.com site history.
Update history to reflect merge of #1693
Update history to reflect merge of #1695
Update history to reflect merge of #1670