:copydot should ignore vim swap files

Seems fairly common to want to `rake deploy` while you might still be
editing a blog post in vim (which creates hidden .swp files). If you
attempt to do so, you'll trip on OSX with the following:

  cp -r source/_posts/.2013-12-09-test.markdown.swp public/_posts/.2013-12-09-test.markdown.swp
  rake aborted!
  No such file or directory - public/_posts/.2013-12-09-test.markdown.swp

Instead, let's try to avoid copying .swp files.

See also: commit 254cdd35