shill: Add IPAddress class

Add class that holds IP Addresses.  Routing will need these.
Move address family constants out of IPConfig to this new class.

BUG=chromium-os:17277
TEST=New unittest

Change-Id: I69d9c4d551061de890ed6919462597a15ae51857
Reviewed-on: http://gerrit.chromium.org/gerrit/4180
Reviewed-by: Darin Petkov <petkov@chromium.org>
Tested-by: Paul Stewart <pstew@chromium.org>
diff --git a/byte_string.h b/byte_string.h
index bfba54e..c4c38b2 100644
--- a/byte_string.h
+++ b/byte_string.h
@@ -14,6 +14,7 @@
 // Provides a holder of a string of bytes
 class ByteString {
  public:
+  ByteString() {}
   ByteString(const ByteString &b) : data_(b.data_) {}
   explicit ByteString(size_t length) : data_(length) {}
   ByteString(const unsigned char *data, size_t length)