* make FIRDocumentListenOptions.includeMetadataChanges private
as discussed in Firestore API discussion, the name clashes creates confusion. Android SDK already not exposing the property; here change iOS SDK to match it.
* update CHANGELOG
* Replacing FIR_SWIFT_NAME macro with NS_SWIFT_NAME.
This pushes the minimum Xcode version to 7.3, as NS_SWIFT_NAME was
limited before that version (which is why the macro was introduced in
the first place).
* Fixed FIRMessaging header
zxu