Added very trivial progress indication support

Hi,

This one I also added to my own copy of the file, but I want to contribute it back to you so I can do NuGet updates. :smile: It should be a no-brainer really; you get access to an event handler which will get called by Unzip.cs as the unzipping proceeds.

(As a slight side note, the crc32 code you added could take some cleanups I think. Not criticizing it as so, just seeing "room for improvement". I am not volunteering at the moment for lack of time, but it feels like the current implementation strongly breaks the [separation of concerns](http://en.wikipedia.org/wiki/Separation_of_concerns) design principle, where all the actual code for calculating the CRC32 is inline in the actual unzipping code. It would be better to try to keep that part in a separate method, or even a sub-class of its own wouldn't be improper in this case.)

Best regards,
Per