Generate model Tag.  #8, #15

$ rails generate model tag name:string post:references

After generation, do

$ rake db:migrate

Signed-off-by: ataka <masayuki.ataka@gmail.com>
Print test message in documentation format (RSpec).  #8

Signed-off-by: ataka <masayuki.ataka@gmail.com>
Simple RSpec test for model post.  #8

Signed-off-by: ataka <masayuki.ataka@gmail.com>
Refactoring post_spec.rb.  #8

:title validation seems wrong.
It should be fixed.

ref. http://d.hatena.ne.jp/t-wada/20100228/p1

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