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/Makefile b/Makefile
index f75e836..a1b8c38 100644
--- a/Makefile
+++ b/Makefile
@@ -889,6 +889,7 @@
     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 \
@@ -2018,6 +2019,7 @@
     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 \