add <ul> to make valid html
rework summary attribute

*   write summary property in modern style
*   introduce get_summary which is memoizable, based on localsiteurl
*   introduce get_siteurl
add paragraph mentioning markdown extensions to docs
Document Markdown inclusion in Quickstart install
Merge pull request #1358 from ingwinlu/md_exten_add

Improve docs regarding Python-Markdown extensions
Merge pull request #1361 from ingwinlu/rewritten_summary_2

Re-work summary attribute to be memoizable
Warn that signal receivers are weakly-referenced

For the signal get_generators in particular, it may seem natural to use
a lambda or inner function as the signal receiver, but this does not
work as the receiver is collected before it can be called.
Merge piwik.html into analytics.html

Addresses #1512.
Merge pull request #1518 from iKevinY/merge-analytics

Merge piwik.html into analytics.html
Merge pull request #1457 from svenkreiss/master

Add <ul> to authors.html template so HTML is valid