Ruby » Ruby master

Prepare to make CSV module to default gem.

• lib/csv.gemspec: initial gemspec for csv gem.
• tool/rbinstall.rb: support gemspec located under lib direcotry like lib/foo.gemspec

[Feature #13177]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@57491 b2dd03c8-39d4-4d8f-98ff-823fe69b080e