[Constants] Add PROXY_TYPE for PBXContainerItemProxy#proxy_type
[Spec][Fixtures] Add a project with a subproject
Add PBXContainerItemProxy#remote?
Add PBXNativeTarget#dependency_for_target
Fix: PBXNativeTarget#add_dependency for targets from subprojects

The serialized project couldn't been deserialized properly, because the target was written, which caused a warning about an attempt to initialize an object with an unknown UUID.
Add PBXContainerItemProxy#proxied_object
[Changelog] Fixed usage of empty lines

Use two empty lines before each version heading, without the first
[Changelog] Add entries for #178
Set name of PBXTargetDependency

As Xcode does this too, in addition to 9351b1d.
This is perhaps needed by Xcode for subproject target dependencies to display the name in red color, if the target dependency's product is missing because the subproject is missing, which would cause that the name couldn't be resolved.
Merge pull request #188 from mrackwitz/fixed_subproject_target_refs

Fixed Subproject Target Dependencies