Note that Cartfiles are a subset of OGDL
Extend Cartfile syntax for branches/tags/commits
Rename PinnedVersion.tag to `commitish`

Definition from the git manpage:

Indicates a commit or tag object name. A command that takes a
<commit-ish> argument ultimately wants to operate on a <commit> object
but automatically dereferences <tag> objects that point at a <commit>.
Update Cartfile.lock parsing test with a SHA
Update TestCartfile with branch pinning
Add VersionSpecifier.GitReference

Currently, this just overrides any other version specifiers given
(unless two differing Git references have been provided).

We may want to do something more intelligent in the future, like
determine the merge-base between the Git specifier and any other
versions specified.
Remove unused latestSatisfyingVersion function
Replace most SemanticVersions with PinnedVersions
Peel Git references during dependency resolution

This ensures that we write a SHA to the Cartfile.lock, not whatever the
given branch/tag name was.
Improve error messaging when a ref isn't found
Fetch with a refspec that gets branch updates

Without fetching +refs/heads/*:refs/heads/* into our bare repo, we
won’t pull down updates to the branches in the original repository.
Note that all version specifiers result in pinning
Bump version number
Merge branch '0.4-development' into revparse-dependency-pinning

Conflicts:
	CarthageKitTests/ResolverSpec.swift
Use comma-separated cases instead of `fallthrough`
Merge pull request #196 from Carthage/revparse-dependency-pinning

Extend Cartfile syntax to support pinning to arbitrary branches/tags/commits
Merge branch 'master' into 0.4-development

Conflicts:
	CarthageKit/Resolver.swift
	CarthageKit/Version.swift
	CarthageKitTests/VersionSpec.swift
	Documentation/Artifacts.md
Add a first draft of a copy-framework command
Read from environment variables
Only strip architectures actually present in the binary