Changed download link from …/zipball/... to .../archive/...

In the case of a release (v1.1.0 for example), the `/archive/` file spits out a nice folder named `fireshell-1.1.0` instead of a folder called `toddmotto-fireshell-2cd313b` that you get from the `/zipball/` link.
Merge pull request #21 from noahbass/new_download_link

Change download link from /archive/ to /tag/