Make resetZoom consistent between x- and y-axes

[danvk]

This makes resetZoom behave in the same way for x- and y-axes. If you call resetZoom or double-click the chart, it will zoom all the way out to show the full range of the data, on both axes.

Previously, it would do this for the x-axis but only for a y-axis that had been zoomed by the user. A y-axis set with g.updateOptions({ valueRange }) would not be reset by a double-click, whereas an x-axis set with g.updateOptions({ dateWindow }) would.

This didn't make much sense and led to confusion/bugs. So this makes a breaking change to clean things up.

The source of this difference was valueWindow. This tracked user-driven y-axis zooms, as distinct from programmatic ones (valueRange). There was no corresponding notion for the x-axis, hence the behavior mismatch.

In the process I removed the isZoomedIgnoreProgrammaticZoom option. If you want the isZoomed method to do something different than it does, dygraphs provides plenty of API for you to work with.

This also adds a yAxisExtremes method to correspond to xAxisExtremes. This gets used when you zoom out with animated zooms and in some tests.

Fixes #524
Fixes #326

[coveralls]

Coverage increased (+0.06%) to 90.117% when pulling c5c8d14 on fix-524 into 623dd1d on master.

[coveralls]

Coverage increased (+0.07%) to 90.134% when pulling 3ba4d1d on fix-524 into 623dd1d on master.