Status

@devoncarew Initial implmentation of git status.

I have exposed API to query the status of a file, and of the whole tree.

Caveats

1) getStatus is still not efficient because I was not able to read the modification time of the file. Is modification time available for chrome.ChromeFileEntry ?

2) Writing the and reading the index is still pending. I will be updating it in this CL. 

3) The fileStatus is currently dependent on the index. A corrupt index will result in corrupting the objectstore. It is non-trivial to fix this right now. I will fix this soon.

Thanks