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/LedgerFormats.cpp

@@ -122,12 +122,15 @@ LedgerFormats::LedgerFormats ()
             << SOElement (sfExpiration,          SOE_OPTIONAL)
             ;
 
-    // All three fields are SOE_REQUIRED because there is always a
+    // All fields are SOE_REQUIRED because there is always a
     // SignerEntries.  If there are no SignerEntries the node is deleted.
     add ("SignerList", ltSIGNER_LIST)
             << SOElement (sfOwnerNode,           SOE_REQUIRED)
             << SOElement (sfSignerQuorum,        SOE_REQUIRED)
             << SOElement (sfSignerEntries,       SOE_REQUIRED)
+            << SOElement (sfSignerListID,        SOE_REQUIRED)
+            << SOElement (sfPreviousTxnID,       SOE_REQUIRED)
+            << SOElement (sfPreviousTxnLgrSeq,   SOE_REQUIRED)
             ;
 }