Add an alternative algorithm for zoomify tier size calculation

Zoomify uses a different algorithm than ol3 to calculate the number of
tiles per tier.
These algorithms are compatible in most cases, but they will lead to
different results with some particular image sizes.

This update provides an option for the zoomify source to use one of
these two algorithms.
Use shorter syntax for integer division by 2
Merge pull request #1731 from felixgirault/ol.source.zoomify

Add an alternative algorithm for zoomify tier size calculation