Add custom overview map example

An example featuring 2 maps, with the second acting as an overview map
that follows the center of the first one, only using the available
components (no control).
New OverviewMap control and example, incomplete

Features a working overview map control in which a ol.Map object is
created and bound to the control map (main map) to fit its extent when
the main map resolution and center change.

The overview map doesn't contain a rectangle representing the current
main map extent, it's navigable where it ideally shouldn't (you would
move the rectangle around instead). It doesn't have a
'collapse/expand' button yet.
Merge branch 'official-master' into overviewmap
Merge branches 'official-master' and 'master'
Update custom overview map example
Fix custom overview map example, build check pass
Fix OverviewMap control, integration-test pass
Update OverviewMap control, box added
Merge branch 'official-master' into overviewmap
Fix OverviewMap control - box
Updated OverviewMap examples, 3 ways to customize
Fix OverviewMap control resetExtent_

resetExtent_ now sets the resolution to a ratio between the min and
max ratio set, making the overview map equally friendly when zooming
in or out after a reset call.
Update OverviewMap control css
Fix OverviewMap control

Do not reset extent if box would fin in overview map, just recenter
instead.
Fix OverviewMap control resetExtent_

The ratio calculated was incorrect, now working properly
Fix OverviewMap control, no map interactions
Fix OverviewMap control, build check pass
Merge branch 'official-master' into overviewmap
Fix OverviewMap example title
Update OverviewMap control, layers property

You can define the layers the overview map should use using the layers
property.  If not set, then the main map layers are used (bound).