Add typescript typings

[HaaLeo]

Add the typings like described here

declare module 'gps-time' {
    export function toGPSMS(unixMS: number): number;

    export function toUnixMS(gpsMS: number): number;
}

to support typescript usage.

[davidcalhoun]

Thanks for the idea, I'm totally open to this! Will see if I can add it when I get a chance.