Coverage support.

- open source CoverageCommand.java
- add a collect-coverage.sh script
- update test-setup.sh to be compatible with the coverage collector
- update StandaloneTestStrategy to provide the necessary env variables
- update StandaloneTestStrategy to set the right command line for coverage
- add support for C++ coverage

An HTML report can then be generated with genhtml like this:
genhtml -o report/ -p "$(readlink -f bazel-<project>)" path/to/coverage.dat

Progress on #1118.

--
MOS_MIGRATED_REVID=140125715
diff --git a/tools/coverage/BUILD b/tools/coverage/BUILD
new file mode 100644
index 0000000..2b8bb73
--- /dev/null
+++ b/tools/coverage/BUILD
@@ -0,0 +1,16 @@
+package(default_visibility = ["//visibility:public"])
+
+filegroup(
+    name = "coverage_support",
+    srcs = ["collect-coverage.sh"],
+)
+
+filegroup(
+    name = "coverage_report_generator",
+    srcs = ["dummy_coverage_report_generator"],
+)
+
+filegroup(
+    name = "srcs",
+    srcs = glob(["*"]),
+)