Ubuntu system image 2.1

2.1 (2014-02-20)
================
 * Internal improvements to SignatureError for better debugging. (LP: #1279056)
 * Better protection against several possible race conditions during
   `CheckForUpdate()` (LP: #1277589)
   - Use a threading.Lock instance as the internal "checking for update"
     barrier instead of a boolean. This should eliminate the race window
     between testing and acquiring the checking lock.
   - Put an exclusive claim on the `com.canonical.SystemImage` system dbus
     name, and if we cannot get that claim, exit with an error code 2. This
     prevents multiple instances of the D-Bus system service from running at
     the same time.
 * Return the empty string from `ApplyUpdate()` D-Bus method. This restores
   the original API (patch merged from Ubuntu package, given by Didier
   Roche). (LP: #1260768)
 * Request ubuntu-download-manager to download all files to temporary
   destinations, then atomically rename them into place. This avoids
   clobbering by multiple processes and mimics changes coming in u-d-m.
 * Provide much more detailed logging.
   - `Mediator` instances have a helpful `repr` which also includes the id of
     the `State` object.
   - More logging during state transitions.
   - All emitted D-Bus signals are also logged (at debug level).
 * Added `-L` flag to nose test runner, which can be used to specify an
   explicit log file path for debugging.
 * Fixed D-Bus error logging.
   - Don't initialize the root logger, since this can interfere with
     python-dbus, which doesn't initialize its loggers correctly.
   - Only use `.format()` based interpolation for `systemimage` logs.
 * Give virtualized buildds a fighting chance against D-Bus by
   - using `org.freedesktop.DBus`s `ReloadConfig()` interface instead of
     SIGHUP.
   - add a configurable sleep call after the `ReloadConfig()`. This defaults
     to 0 since de-virtualized and local builds do not need them. Set the
     environment variable `SYSTEMIMAGE_DBUS_DAEMON_HUP_SLEEP_SECONDS` to
     override.
  * Run the tox test suite for both Python 3.3 and 3.4.