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
minor spelling fixes
fix the parameters for a logging.warning call
fix missing tab in Makefile template
Merge pull request #1511 from olivierverdier/makefile_tab

Fix missing tab in Makefile template
Make StaticGenerator skip content sources. Refs #1019.

This change partially addresses issue #1019, by teaching Pelican to distinguish
between static files and content source files. A user can now safely add the
same directory to both STATIC_PATHS and PAGE_PATHS (or ARTICLE_PATHS). Pelican
will then process the content source files in that directory normally, and
treat the remaining files as static, without copying the raw content source
files to the output directory. (The OUTPUT_SOURCES setting still works.)

In other words, images and markdown/reST files can now safely live together.

To keep those files together in the generated site, STATIC_SAVE_AS and
PAGE_SAVE_AS (or ARTICLE_SAVE_AS) should point to the same output directory.

There are two new configuration settings:

STATIC_EXCLUDES=[]  # This works just like PAGE_EXCLUDES and ARTICLE_EXCLUDES.
STATIC_EXCLUDE_SOURCES=True  # Set this to False to get the old behavior.

Two small but noteworthy internal changes:

StaticGenerator now runs after all the other generators. This allows it to see
which files are meant to be processed by other generators, and avoid them.

Generators now include files that they fail to process (e.g. those with missing
mandatory metadata) along with all the other paths in context['filenames'].
This allows such files to be excluded from StaticGenerator's file list, so they
won't end up accidentally published. Since these files have no Content object,
their value in context['filenames'] is None. The code that uses that dict has
been updated accordingly.
Merge pull request #1509 from foresto/static-files-anywhere

Make StaticGenerator skip content sources. Refs #1019.
Make PAGE_EXCLUDES work with subdirs. Fixes #1500.

The old code was naively comparing the strings in PAGE_EXCLUDES to the
subdirectory names produced by os.walk(). (Same with ARTICLE_EXCLUDES.)
This had two surprising effects:

Setting PAGE_EXCLUDES=['foo'] would exclude all directories named foo,
regardless of whether they were in the top-level content directory or
nested deep within a directory whose contents should not be excluded.

Setting PAGE_EXCLUDES=['subdir/foo'] would never exclude any directories.

In other words, there is no way to exclude a subdirectory without risking
the accidental exclusion of other directories with the same name elsewhere
in the file system.

This change fixes the problem, so 'subdir/foo' and 'foo' will be distinct
and both work as expected. If anyone out there is depending on the old
behavior, they will have to update their settings. I don't expect it to
affect most users yet, since Pelican doesn't yet make nested directory
structures very useful. When it does, this fix will become important to
more people.
rework summary attribute

*   write summary property in modern style
*   introduce get_summary which is memoizable, based on localsiteurl
*   introduce get_siteurl
Merge pull request #1501 from foresto/subdir-excludes

Make PAGE_EXCLUDES and ARTICLE_EXCLUDES work with subdirs. Fixes #1500.
Merge pull request #1466 from cgar/spelling

Minor spelling fixes
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.