Key import and export

Introduces two new RPC calls:
* dumpprivkey: retrieve the private key corresponding to an address
* importprivkey: add a private key to your wallet

The private key format is analoguous to the address format. It is
a 51-character base58-encoded string, that includes a version number
and a checksum.

Includes patch by mhanne:
* add optional account parameter for importprivkey, if omitted use default

src/makefile.mingw

@@ -70,6 +70,7 @@ OBJS= \
     obj/net.o \
     obj/protocol.o \
     obj/bitcoinrpc.o \
+    obj/rpcdump.o \
     obj/script.o \
     obj/util.o \
     obj/wallet.o