Misc OSX porting things:

* autostart the assistant on OSX, using launchd **done**
* icon to start webapp **done**
* Use OSX's "network reachability functionality" to detect when on a network
  <http://developer.apple.com/library/mac/#documentation/Networking/Conceptual/SystemConfigFrameworks/SC_Intro/SC_Intro.html#//apple_ref/doc/uid/TP40001065>
* Fix app build to work on more OSX versions.

Gripes:

* The assistant has to wait a second when a new file is created,
  to work around some bad behavior when pasting a file into the annex.
  [[details|bugs/pasting_into_annex_on_OSX]]. That's one more second
  before the file is synced out.