Introduce the (outside-of-iomgr) pollset API.

This CL introduces the public side of this interface. There will need to be an
iomgr-private API also, but this will be a per-implementation API and so is not
covered here.

I've taken care of wiring the interface through the codebase in the manner that
I expect it will be used.
	Change on 2014/12/17 by ctiller <ctiller@google.com>
-------------
Created by MOE: http://code.google.com/p/moe-java
MOE_MIGRATED_REVID=82376987
diff --git a/build.json b/build.json
index 72c74d9..a970172 100644
--- a/build.json
+++ b/build.json
@@ -119,6 +119,7 @@
         "src/core/iomgr/endpoint_pair_posix.c",
         "src/core/iomgr/iomgr_libevent.c",
         "src/core/iomgr/iomgr_libevent_use_threads.c",
+        "src/core/iomgr/pollset.c",
         "src/core/iomgr/resolve_address_posix.c",
         "src/core/iomgr/sockaddr_utils.c",
         "src/core/iomgr/socket_utils_common_posix.c",
@@ -215,6 +216,7 @@
         "src/core/iomgr/iomgr_completion_queue_interface.h",
         "src/core/iomgr/iomgr.h",
         "src/core/iomgr/iomgr_libevent.h",
+        "src/core/iomgr/pollset.h",
         "src/core/iomgr/resolve_address.h",
         "src/core/iomgr/sockaddr.h",
         "src/core/iomgr/sockaddr_posix.h",