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".