Tree-view with nested git repo support

Related pull-request: atom#469

Works quite well even with >200 nested .git repos, though with some limitations:

1. If there are a lot of nested. git repos then start-up time might noticeably increase(with ~200 repos it was 10 sec). The reason is that GitRepository object seems to be not fully async while refreshing status.
2. Status refresh of GitRepositry objects on window_focus event is disabled, so if repos changed out of the Atom it might reflect wrong status.

Installation

• Unpack contents to ~\.atom\packages\tree-view
  (on windows C:\Users\<username>\.atom\packages\tree-view
• Re-open Atom