Update BUILD_OSX.md

build_unix instead of build_linux
Clean up last night's hack preventing us from requesting the same item from peers several times
Merge pull request #59 from cassiopaia/patch-2

Update BUILD_OSX.md
Begin work on "alert" messages.
Add RPC calls to track block/transaction propagation times.
Track when we sent and received the last message to peers, and report it through JSON RPC like bitcoin does.
Abstract out the code for scanning the network topology during tests, and print out more statistics on data, blocks, and transactions generated during testing.
Merge pull request #27 from BitShares/master

Merge from Remote:BitShares