Refactored progress reporting infrastructure

The progress package contains functionality to deal with progress
reporting. The functionality is built to serve progress reporting for
infrastructure operations when talking the vSphere API, but is generic
enough to be used elsewhere.

At the core of this progress reporting API lies the Sinker interface.
This interface is implemented by any object that can act as a sink for
progress reports. Callers of the Sink() function receives a send-only
channel for progress reports. They are responsible for closing the
channel when done. This semantic makes it easy to keep track of multiple
progress report channels; they are only created when Sink() is called
and assumed closed when any function that receives a Sinker parameter
returns.
Merge pull request #101 from pietern/progress

Refactored progress reporting infrastructure