A way to track the progress of an async file upload and to cancel an async file upload

[nahi]

From an user: (Thanks!)

I've incorporated httpclient into one of my projects, and needed to add
a few features:

- A way to track the progress of an async file upload
- A way to cancel an async file upload
- A few more extensions for the default MIME handler

Connection#cancel cancels an ongoing upload, and Connection#written can
be used to ask the connection object how much data has been transferred
so far, in bytes.

I have gzipped and attached the output of 'svn diff'.