Refactor fee rules to make them actually readable.

This (nearly) doesn't change fee rules at all:
 * To make it into the fee transaction area, the dPriority comparison
   changed from < to <=
 * We now just ignore transactions > MAX_BLOCK_SIZE/4 instead of
   doing some calculations to require increasingly large fees as
   size increases.

src/wallet.cpp

@@ -1301,7 +1301,7 @@ bool CWallet::CreateTransaction(const vector<pair<CScript, int64> >& vecSend,
                 // Check that enough fee is included
                 int64 nPayFee = nTransactionFee * (1 + (int64)nBytes / 1000);
                 bool fAllowFree = AllowFree(dPriority);
-                int64 nMinFee = GetMinFee(wtxNew, 1, fAllowFree, GMF_SEND);
+                int64 nMinFee = GetMinFee(wtxNew, fAllowFree, GMF_SEND);
                 if (nFeeRet < max(nPayFee, nMinFee))
                 {
                     nFeeRet = max(nPayFee, nMinFee);