Auto merge of #1105 - bitcartel:zc.v0.11.2.z6_issue_424, r=ebfull

New private/public key pairs for broadcasting alert messages

Implements #424

Fixes and integrates method of sending alerts as described by upstream here:
- https://gist.github.com/laanwj/0e689cfa37b52bcbbb44

To send an alert:
- Copy private keys into alertkeys.h.
- Modify alert parameters and message found in sendalert.cpp
- Build and run to send the alert e.g. ./zcashd -printtoconsole -sendalert

Tested and verified with local nodes on alpha 6 testnet.

src/Makefile.am

@@ -188,6 +188,7 @@ libbitcoin_util_a-clientversion.$(OBJEXT): obj/build.h
 # server: shared between bitcoind and bitcoin-qt
 libbitcoin_server_a_CPPFLAGS = $(BITCOIN_INCLUDES) $(MINIUPNPC_CPPFLAGS)
 libbitcoin_server_a_SOURCES = \
+  sendalert.cpp \
   addrman.cpp \
   alert.cpp \
   bloom.cpp \