Remove IsFromMe() check in CTxMemPool::accept()

Fixes issue #2178 : attacker could penny-flood with invalid-signature
transactions to deduce which addresses belonged to your node.

I'm committing this early for code review; I still need to write up
a test plan.

Executive summary of fix: check all transactions received from the network
for penny-flood rate-limiting before adding to the memory pool. But do NOT
ratelimit transactions added to the memory pool:
  - because of blockchain reorgs
  - stored in the wallet and added at startup
  - sent from the GUI or one of the send* RPC commands (CWallet::CommitTransaction)

The limit-free-transactions code really should be a method on CNode, with
counters per-peer. But that is a bigger change for another day.
Let limitfreerelay=0 reject ALL free transactions
spendfrom.py sends coins from a particular address or addresses

Should be both useful and also a pretty good demonstration of
using the raw transactions API.
Check only 288 blocks at startup by default
Merge pull request #2222 from sipa/check288

Check only 288 blocks at startup by default
Merge pull request #2182 from gavinandresen/addressoracle

Remove IsFromMe() check in CTxMemPool::accept()
Merge pull request #2162 from gavinandresen/spendfrom

spendfrom.py sends coins from a particular address or addresses