Add Jacoco code coverage

After running tests, you can run jacocoTestReport. The jacocoTestReport
task does not depend on the tests, so they should be run separately.

There is still a lot of noise in the jacoco output since we aren't yet
filtering generated code.
diff --git a/build.gradle b/build.gradle
index 546d3c2..b1b5fdb 100644
--- a/build.gradle
+++ b/build.gradle
@@ -15,6 +15,7 @@
     apply plugin: "idea"
     apply plugin: "osdetector"
     apply plugin: "signing"
+    apply plugin: "jacoco"
 
     group = "io.grpc"
     version = "0.1.0-SNAPSHOT"