Remove include of windows.h from allocators.h

Create an allocators.cpp, and move all of the #ifdef WIN32
code and the #include of windows.h into it.

Two motives for this cleanup:
1. I'm getting a weird error in windows.h in my smartfee branch.
2. allocators.h is included (indirectly) just about everywhere, so
this should speed up Windows compiles quite a lot.

src/Makefile.am

@@ -31,7 +31,7 @@ obj/build.h: FORCE
 	  $(abs_top_srcdir)
 version.o: obj/build.h
 
-libbitcoin_a_SOURCES = addrman.cpp alert.cpp bitcoinrpc.cpp bloom.cpp \
+libbitcoin_a_SOURCES = addrman.cpp alert.cpp allocators.cpp bitcoinrpc.cpp bloom.cpp \
   chainparams.cpp checkpoints.cpp core.cpp crypter.cpp db.cpp hash.cpp \
   init.cpp key.cpp keystore.cpp leveldb.cpp main.cpp miner.cpp \
   netbase.cpp net.cpp noui.cpp protocol.cpp rpcblockchain.cpp rpcdump.cpp \