Add full functionality for XHR from local fs

[gideonthomas]

This primarily involves two things:

• the XHR shim that is implemented in brackets-browser-livedev here relies on event listeners attached to the XHR object (onerror, onreadystatechanged, etc.). We need to deal with the case where addEventListener is used instead.
• getAllResponseHeaders is not implemented for the XHR shim and it needs to be implemented so that we return relevant headers to the process of local fs XHR

[humphd]

We can probably simulate the addEventListener and removeEventListener stuff with https://github.com/humphd/brackets/blob/bramble/src/utils/EventDispatcher.js

[humphd]

I think for getAllResponseHeaders we can just have a simple set of things we return, like Date, Content-Type, Content-Length, Server and that's probably enough?

[gideonthomas]

We also need to deal with the Accept header; if it is set, we should try to serve it as the requested MIME type?

[humphd]

I think solving this fully is a rabbit hole we should be careful we don't fall into. Let's deal with the common cases, which I'd argue are "whatever jquery does" and leave it at that.

[humphd]

With #549 I need to remember to not use this shim for doing XHR with SW.