minor spelling fixes
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.
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