Cleans up a few now unnecessary methods.

Starts using only requires.
Allows Entry to publish Atom.
Tests OStatus::Entry.
Tests OStatus::Feed.
Adds to_atom to Feed that uses the Atom generator.

There are some issues with this approach that I'm not a fan of. But at
least I'm isolating the atom generation to a class.

For instance, Entries are printed as their #inspect.
Allows Atom entries to be correctly encoded in Feed's to_atom.
Renames Entry#info to Entry#to_hash.
Adds tests for Entry and Feed for defaults.
Tests PortableContacts#initialize.
Adds #to_hash to PortableContacts.
Adds Atom generator classes.
Tests Activity.
Adds docs to .gitignore.
Tests Author.
Adds Link.
Adds Feed#to_link to generate a Link object that refers to the feed.
Adds *.rbc to .gitignore for rubinius.
Tests Thread.
Adds in_reply_to field to Entry.
Adds Entry#to_link.