Replace rakegem

Rakegem isn't really getting us much. This commit replaces the generated gemspec with a simpler one. The `rake {build,release}` tasks have been moved to `script/{package,release}`.

This is basically a direct copy from [Faraday's gemspec](https://github.com/lostisland/faraday/blob/master/faraday.gemspec) and [build scripts](https://github.com/lostisland/faraday/blob/master/script/release).