Updates docs for Python 3

In Python 3, the simpleHTTPServer has moved into the http module, so the command `python -m simpleHTTPServer no longer works. This minor change adds instructions for those of us using Python 3.
add <ul> to make valid html
Enables custom locale while feed generation.

Fixes wrong language of month names in feed links.

Related to #1198
Updates the custom_locale test feed files to match the correct month names
fix the parameters for a logging.warning call
Add tests for abbr_role. Refs #949
Fix bug with custom reST :abbr: role. Fixes #949

This fixes things so that newlines are allowed within the explanation of
an :abbr: role in reST mark-up.
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
Minor documentation improvements
Mention the INDEX_SAVE_AS setting in FAQ and direct templates

This setting can be particularly useful when making a static homepage.
add notices about caching interfering with changes to settings and plugins
Merge pull request #1499 from matrixise/fix_logger

Fix the parameters for a logging.warning call