AU: Beginnings of delta support

AU: Beginnings of delta support

- proto file for delta files; still needs hardlink support

- code to generate a delta update from two directory trees (old, new).

- code to parse delta update

- Actions: postinst-runner, install, bootable flag setter, filesystem
 copier, Omaha response handler, Omaha request preparer,

- misc utility functions, like StringHasSuffix(), templatized Action
 classes to feed/collect an object from another action.

- FilesystemIterator: iterates a directory tree with optional
 exclusion path. Tolerates deleting of files during iteration.

- Subprocess class: support for synchronously or asynchronously
 running an external command. Doesn't pass any env variables.

- Integration test that strings many Actions together and tests using
 actual Omaha/Lorry. Currently only tests full updates.

- New simple HTTP server for unittest that supports fake flaky
 connections.

- Some refactoring.

Committed: http://chrome-svn/viewvc/chromeos?view=rev&revision=334

[adlr, 2009-12-04 18:45:56]

I've been having some trouble w/ my chromium checkouts, so I gave up and created
a new one. I really think this time I'll actually be able to check it in. This
is the same code review from before.

[Daniel Erat, 2009-12-04 18:53:49]

LGTM based on previous review