Add FactCursors and application fact datasources

Summary:

• Add PhabricatorApplication. This is a general class that I have grand designs for, but used here to allow applications to provide objects for analysis by the facts appliction.
• Add FactCursors, to keep track of where iterators are.
• Make the daemon do something sort of useful.
• Add bin/fact cursors for showing and managing objects and cursors.
• Add some options to bin/fact analyze.

Test Plan:

• bin/fact cursors, bin/fact cursors --reset DifferentialRevision, bin/fact cursors --reset X
• bin/fact analyze, bin/fact analyze --all, bin/fact analyze --iterator DifferentialRevision --skip-aggregates
• bin/phd debug fact

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T1562

Differential Revision: https://secure.phabricator.com/D3098