Introduce SpecFetcher proxy.

Based on jstrzibny's work.
Introduce Package proxy.
Introduce Format proxy.
RubyGems 2.0 compatibility.

Based on jstrzibny's work.
Fix -d option with RubyGems 2.0.

Based on jstrzibny's work.
Update documentation.

* Add package variable for template.
* Note that format is now deprecated.
Update templates to match the changes in source code.
Correctly return URLs for older releases of gems.

Interestingly, this bug went unnoticed for almost 5 years.
Fix source url for RubyGems 1.8 compatibility.
Drop support for RubyGems older than 1.0.0.

EPEL5 has RubyGems 1.2.0+ since 2009. So we can be reasonable sure that
nobody cares about older RubyGems.
Version bump to 0.9.0.