In some cases, for example when using a `ol.control.ZoomSlider`,
that control used to not work at all, because it re-renders the
map and use the size of the elemement to calculate size values.
While activating GoogleMaps, the ol3 map becomes in limbo, thus
caused the size to return `NaN`.
This patch makes sure that the ol3 map is ready before activating
Google Maps.
This patch introduces documentation about the limitations of this
library, notably the 'rotation' that should not be enabled, as
mentioned in \#4.
A new `LIMITATIONS.md` file was added to document this.
A new utility method that returns the default interactions that
include only the ones that are recommended
(well, more like 'exclude' those that aren't).
