Added a functional spec for telling the user that the Pods cannot be updated unless they are installed

e.g. rake 'spec:functional[command/project]'
The single quotes are needed for zsh.

Because it improves the user convenience and bundler does it the same way.

Refactoring: Replaced Installer::Analyzer's getter and setter update_mode by #update, #update= and #update_mode?

Changed signature doc of Pod::Project#run_install_with_update and call it appropriately by Pod::Project::Update#run

Updated CHANGELOG for Core/pull/84

Thanks @dlackty

Conflicts:
	lib/cocoapods/generator/xcconfig/aggregate_xcconfig.rb
	spec/unit/generator/xcconfig/aggregate_xcconfig_spec.rb

Try to fix spec failure on master

Updated CHANGELOG for #1978