move public methods to top

seems in line with conventions in rest of project
Move gem mirror code to be consistent with other features

Factors the gem mirror code similarly to the local git repos feature.

Questions:
* RubygemsMirror class does very little other than provide a static
  location to replace URIs with the desired mirror. Should it exist?
* URI normalization is duplicated. Is there a good single location?
* Validation of mirror URIs occurs in Settings, which contains no other
  validation code. Is there a more desirable location for this?
* Is there a better solution for case-sensitive URIs?
add specs for gem mirror support
add more exposition to mirror config manpage