Add tvOS support to Firestore (#2662)

* Create tvOS host app

* Standard project edits

  * Remove code signing
  * Force C99/C++11
  * Adjust plist file for new location
  * header search paths
  * LDFLAGS

* Add Firestore_{Tests,IntegrationTests}_tvOS

* Add tvOS tests to the Podfile

* Add Firestore tvOS xcodebuild to build.sh

* Teach sync_project to keep tvOS up-to-date

* Add Firestore tvOS to Travis

Firestore/Example/GoogleBenchmark.podspec

@@ -35,6 +35,7 @@ Google's C++ benchmark framework.
 
   s.ios.deployment_target = '8.0'
   s.osx.deployment_target = '10.10'
+  s.tvos.deployment_target = '10.0'
 
   s.requires_arc = false