nodegit: fixed Signature instance variable types
nodegit: fixed Signature instance variable types

See https://github.com/nodegit/nodegit.github.com/issues/52
AgentME/DefinitelyTyped,georgemarshall/DefinitelyTyped,AgentME/DefinitelyTyped,rolandzwaga/DefinitelyTyped,magny/DefinitelyTyped,borisyankov/DefinitelyTyped,georgemarshall/DefinitelyTyped,dsebastien/DefinitelyTyped,mcliment/DefinitelyTyped,AgentME/DefinitelyTyped,AgentME/DefinitelyTyped,rolandzwaga/DefinitelyTyped,magny/DefinitelyTyped,borisyankov/DefinitelyTyped,georgemarshall/DefinitelyTyped,markogresak/DefinitelyTyped,georgemarshall/DefinitelyTyped,one-pieces/DefinitelyTyped,dsebastien/DefinitelyTyped