Extract Rails extension to have matcher

https://github.com/rspec/rspec-rails/issues/808
Moved dependency declaration and removed empty module decl
Use pessimistic activesupport version for ruby < 1.9.3
Fix alias_method_chain to use newer method names
Reduce duplication in message formatting
Add aliases for 2-99 compatibility
Fix Ruby 1.8.x compatibility

instance_methods returns an array of symbols in Ruby 1.9+
but before that it was an array of strings.
Cleanup based on PR feedback
Change rails detection, delete rails.rb
Merge pull request #5 from rspec/rails-have-extensions

Extract Rails extension to have matcher