Fixed compatibility issue with TypeScript 1.8

[kat3su]

Get accessors only will create readonly annotation when compiled with typescript 2.0 and doesn't work well with typescript 1.8

So I added Set accessors as well to avoid the readonly tag.

You can possibly get rid of the get accessors all together as I coudln't see it being used anywhere. But instead, use function. Eg

public getCurrentYear() {}
public getCurrentMonth() {}
public getCurrentDay() {]
public today() {}

[kat3su]

Issue #24

[allenhwkim]

@Krisa could your review this?

[allenhwkim]

I think it's better to remove getter and setter by changing getters to a function instead.