Add bazel Support (#724)

Example of how someone might consume yaml-cpp with bazel:

cc_binary(
    name = "example",
    srcs = ["example.cc"],
    deps = ["@com_github_jbeder_yaml_cpp//:yaml-cpp"],
)

BUILD.bazel

@@ -0,0 +1,14 @@
+cc_library(
+    name = "yaml-cpp_internal",
+    visibility = ["//:__subpackages__"],
+    strip_include_prefix = "src",
+    hdrs = glob(["src/**/*.h"]),
+)
+
+cc_library(
+    name = "yaml-cpp",
+    visibility = ["//visibility:public"],
+    strip_include_prefix = "include",
+    hdrs = glob(["include/**/*.h"]),
+    srcs = glob(["src/**/*.cpp", "src/**/*.h"]),
+)