Support <NSSecureCoding> in MTLModel+NSCoding
++documentation
++error messages
Check all encodable property keys, not just +propertyKeys
Test +allowedClassesByPropertyKey
Add a test XPC service to Mantle Mac Tests target
Check -respondsToSelector: before invoking -methodForSelector:
Share the MantleTestService build scheme
Build harder?
Just pressing keys
Revert "Just pressing keys"

This reverts commit 0117a79dd902dd8454187ff549a0dd2675dd413a.
Revert "Build harder?"

This reverts commit 282bb092ed8ad4226d0374078bca2fcf8f80a7c2.
Revert "Share the MantleTestService build scheme"

This reverts commit f71f8381e5bee1a0096de8aff7c93a23d8b91b48.
Revert "Add a test XPC service to Mantle Mac Tests target"

This reverts commit c3a75e17070453957f36139434804a42d12d8768.

Conflicts:
	Mantle.xcodeproj/project.pbxproj
Explicitly conform to NSSecureCoding
Override +supportsSecureCoding
Remove assertion for allowedClasses
Subclasses may choose to disable MTLModel's archiving for select properties and override -initWithCoder: and -encodeWithCoder: themselves to provide their own encoding/decoding functionality. This assertion fails if they do that.
Decode model version using secure coding
Code style fixes
Make NSSecureCoding opt-in