Drastically improve perf with memoize

I was using moment by itself for a project and then added moment-timezone and noticed a huge performance degradation. After some profiling and digging I noticed that many of the moment-timezone functions could and should be memoized. After applying this patch to my own project, performance returned to how it was pre-moment-timezone. The test speed is also very telling...

**before**
```bash
>> 103487 assertions passed (63010ms)

Done, without errors.
```

**after**
```bash
>> 103487 assertions passed (18811ms)

Done, without errors.
```

Furthermore, this speed difference is further increased in real world use cases were most computing is done in a single, relevant timezone.

Thanks for all the work put into this, timezones are a nightmare...