Remove Jeweller

Jeweller was generating an invalid gemspec and bundler provides all the tools needed these days anyway

Mostly it's just copy-pasting around, the only changes are:
- Removed rubygems < 1.2.0 support, I didn't think it would be much of an issue, but it can be added back in if there's any serious objections.
- Moved to https rubygems
- Added Xeroizer::VERSION
- Removed spec.extra_rdoc_files, they weren't actually rdoc files
- Removed test files from spec.files (I copied this code from the rails gemspecs)