Some fixes for threaded

- `common` segments module is now using threading for polling VCS information
- It now handles multiple `.shutdown()` calls (applicable in case some segment is used twice)
- It now is able to restart thread
- You can now disable current behavior in vim (if threaded segment has not yet obtained any value its computation is done in the main thread).
- Fixed #344.

Note: VCS segments and library refactoring that was discussed in #168 will happen in a separate branch, after I deal with #340 and add some features for powerline daemon (which will also close #330 and make addition of such facility easier).