title: SqlAlchemy won't accept datetime.datetime.now value in a DateTime column
body: <p>I should first mention that I'm using SqlAlchemy through Flask-SqlAlchemy. I don't believe this affects the issue but if it does, please let me know.</p>
  
  <p>Here is the relevant part of the error message I'm getting when running the create_all function in SqlAlchemy</p>
  
  <p><code>InterfaceError: (InterfaceError) Error binding parameter 4 - probably unsupported type. u'INSERT INTO podcasts (feed_url, title, url, last_updated, feed_data) VALUES (?, ?, ?, ?, ?)' (u'http://example.com/feed', u'Podcast Show Title', u'http://example.com', '2012-04-17 20:28:49.117000'</code></p>
  
  <p>Here is my model:</p>
  
  <pre><code>class Podcast(db.Model):
      import datetime
      __tablename__ = 'podcasts'
      id = db.Column(db.Integer, primary_key=True)
      feed_url = db.Column(db.String(150), unique=True)
      title = db.Column(db.String(200))
      url = db.Column(db.String(150))
      last_updated = db.Column(db.DateTime, default=datetime.datetime.now)
      feed_data = db.Column(db.Text)
  
      def __init__(self, feed_url):
          import feedparser
  
          self.feed_url = feed_url
          self.feed_data = feedparser.parse(self.feed_url)
          self.title = self.feed_data['feed']['title']
          self.url = self.feed_data['feed']['link']
  </code></pre>
  
  <p>Can someone tell me how I can get this to work? I've also tried the following model but that also doesn't work. Same error.</p>
  
  <pre><code>class Podcast(db.Model):
      import datetime
      __tablename__ = 'podcasts'
      id = db.Column(db.Integer, primary_key=True)
      feed_url = db.Column(db.String(150), unique=True)
      title = db.Column(db.String(200))
      url = db.Column(db.String(150))
      last_updated = db.Column(db.DateTime)
      feed_data = db.Column(db.Text)
  
      def __init__(self, feed_url):
          import feedparser
  
          self.feed_url = feed_url
          self.feed_data = feedparser.parse(self.feed_url)
          self.last_updated = datetime.datetime.now()
          self.title = self.feed_data['feed']['title']
          self.url = self.feed_data['feed']['link']
  </code></pre>