Add support for a '.gemignore' file

[l8nite]

We've done this on a bunch of gems lately, it would be cool to see it supported in jeweler as an alternative approach to the exclude/includes configuration:

https://gist.github.com/l8nite/18e1aa073d4c758bf440

The advantage with this approach is that the syntax/etc utilizes .gitignore syntax while applying the results to which files are included in your gem.