Introduce a TWO_DAILY tick spacing

Now that we generate correctly-spaced ticks, it's much more likely that we'll generate only a single tick (or no ticks) due to the big gap between `DAILY` and `WEEKLY` spacing. This introduces `TWO_DAILY` tick spacing to ameliorate that.

Note that this would have been a problem even without the change in the default value of `pixelsPerLabel`.

dygraphs v1.0.1:
![dygraphs 1 0 1](https://cloud.githubusercontent.com/assets/98301/5232451/309d6164-771e-11e4-9d50-0aa315cf45ff.png)

dygraphs HEAD:
![head](https://cloud.githubusercontent.com/assets/98301/5232450/309ba3e2-771e-11e4-982a-abaececc96ae.png)

dygraphs after this change:
![two_daily](https://cloud.githubusercontent.com/assets/98301/5232460/582977a4-771e-11e4-9c58-ceeaf18da49e.png)

I would consider this more a new feature than a breaking change, and hence appropriate for inclusion in a 1.1 release.