Add error message print when detecting a double-spend.

When AcceptToMemoryPool fails because it detects a "replacement" /
double-spend, print an error message about it.
Clear mempool when a gametx creates a double-spend.

Remove double-spends from the mempool whenever a new block is attached.
This may fix the long-standing issue with "stuck players" due to kills.
Remove name_clean RPC call.

name_clean is no longer needed, since EraseBadMoveTransactions() (which
was the action of name_clean) is now called on each block anyway.
Bump version number to 1.0.12.
Fix call to EraseBadMoveTranasctions in SetBestChain.

Call EraseBadMoveTransactions *after* updating pindexBest.
Merge pull request #79 from domob1812/pending

Remove double-spends from mempool.