Add mocktracer library. (#47)

* Filling out mocktracer API.

* Fill out StartSpanWithOptions function.

* Add Finish method.

* Fill out MockSpan methods.

* Fill out Log method.

* Add catch2.

* Convert string_view test cases to use catch2.

* Convert value_test to use catch2.

* Convert util_test to use catch2.

* Remove old testing macro.

* Remove custom assert.

* Reorganize.

* Separate out recorder.

* Add InMemoryRecorder.

* Add mocktracer_tracer_test.

* Add binary propagation functions.

* Add other Inject/Extract functions.

* Add code to do base64 conversions.

* Add other Inject/Extract functions.

* Add propagation tests.

* Rework mocktracer log functionality.

* Make mocktracer library follow BUILD_SHARED_LIBS/BUILD_STATIC_LIBS options.

* Make mocktracer build optional.

* Don't install catch2 header.

* Fix recursive macro warning.

* Fix warnings from base64.cpp.

* Log errors.

* Update clang-format script.

* Run clang-format.

* Allow MockTracer to be constructed without a recorder.

* Add mocktracer to bazel build.

* Add mocktracer to bazel build.

* Work around gcc-4.9 limitation.

* Fix installation path.

3rd_party/BUILD

@@ -0,0 +1,20 @@
+cc_library(
+    name = "expected",
+    hdrs = ["include/opentracing/expected/expected.hpp"],
+    strip_include_prefix = "include",
+    visibility = ["//visibility:public"],
+)
+
+cc_library(
+    name = "variant",
+    hdrs = glob(["include/opentracing/variant/*.hpp"]),
+    strip_include_prefix = "include",
+    visibility = ["//visibility:public"],
+)
+
+cc_library(
+    name = "catch2",
+    hdrs = ["include/opentracing/catch2/catch.hpp"],
+    strip_include_prefix = "include",
+    visibility = ["//visibility:public"],
+)