Add Gem.vendor_dir

Part of #943
Add --vendor installation option

This allows install and uninstall from the vendor directory.

Part of #943
Add test for Gem.default_path
Include vendor repository in gem path

Now gems installed with --vendor can be used by RubyGems.  The vendor
directory is only added to the gem path when it exists to reduce both
user confusion and calls to stat(2).

Part of #943
Implement uninstall from the vendor directory

Uninstall warns as the user should be using the package manager
commands to uninstall gems from here.

Part of #943
Add #943 to History