[HooksManager] Show the name of the source for each hook that is run in verbose mode

Huge thanks to @alloy for the `source_location` hint!

The name calculation is wrapped in a lambda so it's not called when it won't be printed out
Allow the specification of plugins in the podfile
[CHANGELOG] Add entry for podfile specification of plugins
[HooksManager] Fix whitelisted plugins hooks not fired

We're incorrectly comparing the hook name instead of the plugin name in
the whitelist
[HooksManager] Change order of arguments for register

“Register slather for post install” vs “Register for post install
slather”
Update integration fixtures
Merge pull request #2830 from CocoaPods/seg-hooks-plugin-name

[HooksManager] Show the name of the source for each hook that is run in verbose mode
[Gemfile] Move Core back to master
[Specs] [Pod::UI] First set of unit specs