Feature/configurable rollback

This is the fix for #99. /cc @mj1856

The defaults are to move ambiguous input backward and invalid input forward.

```js
moment.tz.moveInvalidForward;   // true
moment.tz.moveAmbiguousForward; // false
```

Ambiguous input

```js
moment.tz.moveAmbiguousForward; // false
moment.tz([2012, 10, 4, 0, 30], "America/New_York").format() // 2012-11-04T00:30:00-04:00
moment.tz([2012, 10, 4, 1, 30], "America/New_York").format() // 2012-11-04T01:30:00-04:00
moment.tz([2012, 10, 4, 2, 30], "America/New_York").format() // 2012-11-04T02:30:00-05:00
moment.tz.moveAmbiguousForward = true;
moment.tz([2012, 10, 4, 0, 30], "America/New_York").format() // 2012-11-04T00:30:00-04:00
moment.tz([2012, 10, 4, 1, 30], "America/New_York").format() // 2012-11-04T01:30:00-05:00
moment.tz([2012, 10, 4, 2, 30], "America/New_York").format() // 2012-11-04T02:30:00-05:00
```

Invalid input

```js
moment.tz.moveInvalidForward; // true
moment.tz([2012, 2, 11, 1, 30], "America/New_York").format(); // 2012-03-11T01:30:00-05:00
moment.tz([2012, 2, 11, 2, 30], "America/New_York").format(); // 2012-03-11T03:30:00-04:00
moment.tz([2012, 2, 11, 3, 30], "America/New_York").format(); // 2012-03-11T03:30:00-04:00
moment.tz.moveInvalidForward = false;
moment.tz([2012, 2, 11, 1, 30], "America/New_York").format(); // 2012-03-11T01:30:00-05:00
moment.tz([2012, 2, 11, 2, 30], "America/New_York").format(); // 2012-03-11T01:30:00-05:00
moment.tz([2012, 2, 11, 3, 30], "America/New_York").format(); // 2012-03-11T03:30:00-04:00
```