Simple multisigning (RIPD-182):

With this changeset two-level multisigning is removed from the
codebase and replaced with single-level multisigning.

Additionally, SignerLists in the ledger are prepared for the
possibility of multiple SignerLists per account.  This was done
by adding a defaulted 32-bit SignerListID to each SignerList.
The SignerListIndex calculation incorporates the SignerListID.

There are three known missing elements:

 1. Multisigned transactions should require higher fees than
    regular (single-signed) transaction.  That's not yet
    implemented.

 2. It should be possible to disable the master key on an account
    if that account is multisign enabled (has a signer list).
    That's not yet implemented.

 3. Documentation about multisigning needs to be improved.

Multisigning is still compiled out of the code base.  To enable
multisigning for a stand-alone rippled, change the
RIPPLE_ENABLE_MULTI_SIGN macro (in BeastConfig.h) to "1" and
rebuild.

This commit also addresses:
 o RIPD-912: Remove multisign APIs from STObject, and
 o RIPD-944: Replace common_transactor with jtx at call sites.

src/ripple/protocol/impl/InnerObjectFormats.cpp

@@ -29,15 +29,10 @@ InnerObjectFormats::InnerObjectFormats ()
         << SOElement (sfSignerWeight,         SOE_REQUIRED)
         ;
 
-    add (sfSigningFor.getJsonName ().c_str (), sfSigningFor.getCode ())
-        << SOElement (sfAccount,              SOE_REQUIRED)
-        << SOElement (sfSigningAccounts,      SOE_REQUIRED)
-        ;
-
-    add (sfSigningAccount.getJsonName ().c_str (), sfSigningAccount.getCode ())
+    add (sfSigner.getJsonName ().c_str (), sfSigner.getCode ())
         << SOElement (sfAccount,              SOE_REQUIRED)
         << SOElement (sfSigningPubKey,        SOE_REQUIRED)
-        << SOElement (sfMultiSignature,       SOE_REQUIRED)
+        << SOElement (sfTxnSignature,         SOE_REQUIRED)
         ;
 }