re-work the fileSystem library

This CL is a re-write of the chrome.file_system library. This maps much more closely to the actual chrome.fileSystem API; the previous version was essentially just what I needed for my chrome app. There are some non-API utility methods in here to make it easier to get content into and out of files. Any other thoughts on where this should live are appreciated. I think that in general we'll want some higher level interface to some of the APIs. I.e. map closly to the chrome app APIs, but add some abstractions on top to make them easier to use.

The fileSystem API is difficult to test - getting a FileEntry requires a user action. I'm planning on adding a new example into the example/ directory, with a button to get a file entry and then run through tests. Thoughts welcome!