Add and delete TAGS, in each entry page.  #15

Signed-off-by: ataka <masayuki.ataka@gmail.com>
Rewrite tag join code using View Helper.  close #15

Move it from app/views/posts/show.html.erb
to app/helpers/posts_helper.rb.

Signed-off-by: ataka <masayuki.ataka@gmail.com>