Store feed items' feed

Ref #16
Remove (generated) Content field
Change feed item's description data type to []byte

string data type can only hold up to 500 chars
Add simple login/logout/register/update page
GetFeed also returns the key
Allow users to subscribe to feeds

Closes #17
Show subscription list
Add unit test for utils.Sha1()
Add appengine build constraints

They are still useful, for example to let non-GAE go to run the tests