Fix problem with accepting tx to mempool.

The update to 1.0.13 introduced an issue where transactions spending
unconfirmed outputs were not allowed into the mempool.  This is fixed
now.
Merge pull request #87 from domob1812/fix-pending

Fix problem with accepting tx to mempool.
Use UTXO db to cross-check priority calculation.

Use the UTXO database to look up previous transactions in CreateNewBlock
and compare the resulting data for a tx's priority to the old
calculation.  This is a first step in removing the load from the
blockchain itself and replacing it with *only* the UTXO lookup.
Use only UTXO set to calculate transaction priorities.
Simplify name_show and name_history.

Simplify the implementations of name_show and name_history.  This makes
it possible to get rid of GetValueOfTxPos and some redundant disk loads.
Remove now unused GetNameAddress(CDiskTxPos).
Remove obsolete FIXME remark.
Replace ReadTxIndex by ContainsTx in wallet.cpp.

Replace a call to ReadTxIndex by the "simpler" ContainsTx at one place
where this is all that is actually used.
Remove FetchInputs.

Remove FetchInputs and manually code some logic in signrawtranscation.
The new logic tries to load the tx from the wallet, mapTransactions or
the UTXO set.  No access to the blockchain itself is tried.
Replace GetTransaction by mapTransactions in name_pending.