[post] content_typeのカラムを追加

- t.string   "content_type"
[user/post] 投稿データ・ユーザーの紐付け と 投稿データの設定

## user
  has_many :posts, dependent: destroy

## post
  attr_accessible :date, :filename, :image, :text, :time
  belongs_to :author, class_name: "Uesr", foreign_key: "user_id"
  validates :image, presence: true
  validates :text, length: { maximum: 200 }