Add Pod lib command #850

Tests are still missing.
Merge branch 'master' into lib-subcommand

* master: (59 commits)
  [Bundle] update
  Update CHANGELOG.md
  Release 0.21.0.rc1
  [Gemspec] Update Xcodeproj
  [Bundle] update
  Changelog
  [Bundle] Update
  [Fixtures] Update for release
  [Bundle] Update
  [Rakefile] Examples robustness for future versions of Xcode
  [CHANGELOG]
  [Generator::Xcconfig] Docs
  [Bundle] Use Xcodeproj from master
  [Installer] Rename #targets to #aggregate_targets
  Add files which were creating issues with git
  Delete files creating issues with git
  Update integration test fixtures
  Case change for case-sensitive file systems
  Update tests
  Move method below attributes
  ...

Conflicts:
	Gemfile.lock
Use cp instead of rsync to copy frameworks (refs #1063).

Add mkdir command to make sure the folder Contents/Frameworks exists (otherwise the first framework becomes this folder).
Change rsync command to cp to keep the file structure and all symbolic links.
[Validator] do not assume specific of targets when building file_accessors to test against
[Validator] Added regression test for `undefined method `spec_consumer'`
[Validator] Improve file_accessor determination
[Validator] Improve regression test for `undefined method `spec_consumer'`

- let spec in "validates a podspec with dependencies" validate
- fixed a bug in `write_podspec`
Merge pull request #1130 from HBehrens/master

fix for erroneous linting on 0.21.0.rc1
[CONTRIBUTING] Specifically mention the spec repo.
Remove usage of deprecated ALWAYS_SEARCH_USER_PATHS build setting
CHANGELOG
CHANGELOG
[Generator::Xcconfig] Include the extension in the imported files

See #1152
[Rakefile] More examples robustness for future versions of Xcode
[Generator::Xcconfig] Include the extension in the imported files 2
Release 0.21.0
[Bundle] update
[Gemspec] Include the changelog of RCs in the post install message
[Rakefile] Improve release process
[Installer] Use aggregate target for library reppresentation

Closes #1157