Adds coinbaseflags, mintime, and curtime to output of getmemorypool.
Since users of getmemorypool generate coinbase transactions themselves, they need bitcoind to indicate what should be included in the coinbase data.
Mintime and curtime are useful for generating work by changing the timestamp. Without them, P2Pool can only increment the timestamp. With the knowledge of the minimum allowable timestamp, work can be generated symmetrically around the current timestamp, reducing future bias.
Generation (without using getmemorypool) tested on testnet: http://blockexplorer.com/testnet/block/00000000014be554b29d072c2c079f8e120715d2799899cc9774a8dd02c78345
