Add table_prefix option for jekyll-importer

Add table_prefix to use with WordPress and Joomla importers so they can modify the default value.
Add Asset Path Tag to plugin list in docs

Requested by @mattr-
https://github.com/mojombo/jekyll/issues/881#issuecomment-27276509
Update implementation of listen for v1.3.x
Merge pull request #1687 from mojombo/fix-listen

Fix implementation of listen for v1.3.x
Update history to reflect merge of #1687
updated end to endfor
Merge pull request #1691 from katydecorah/master

updated end to endfor
Update history to reflect merge of #1691
Prep for the 1.3.0 release
Plugin description updates for Monthly/Category archive generator
Bump version to 1.3.0
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.
Minor fixes to v1.3.0 release post.
Fix release date of v1.3.0 in History file
Fix link to /docs/datafiles/ in 1.3.0 release post.
Merge pull request #1694 from mojombo/release-1.3.0

Release 1.3.0
Release 1.3.0
Release 1.3.0
Update jekyllrb.com site history.
Extract Site#filter_entries into its own class