MessageProcessor: Use raw zmq socket

makes transition to CurveZMQ easier

removes some cron and timeout functionality
simplify checks for cron time
moved cron timer to OTCron class
expose cron timing request
introduce event loop in server
Merge pull request #461 from mhamrle/eventloop

Use raw zmq socket, re-enable cron
Extended comment about cron/timing.
Merge pull request #463 from benma/develop

Extended comment about cron/timing.
OTInstrument: rename class to Instrument
OTInstrument: rename files to Instrument.{h,c}pp
Merge pull request #465 from toxeus/renameOTInstrument

OTInstrument: rename to Instrument
Rename: Server ID -> Notary ID.

Rename SERVER_NYM_ID -> NOTARY_NYM_ID.
Rename "User ID" -> "Nym ID".
Merge pull request #466 from benma/develop

Rename missed instances of Server ID and User ID.
OTItem: rename class to Item
OTItem: rename files to Item.{h,c}pp
Merge pull request #468 from toxeus/renameOTItem

OTItem: rename to Item
OTLedger: rename class to Ledger
OTLedger: rename files to Ledger.{h,c}pp
Merge pull request #469 from toxeus/renameOTLedger

OTLedger: rename to Ledger