overview map uses projection parameter or base projection before defaults

Addresses #2998:
- creates a view object using options.projection if specified, default EPSG:3857 otherwise
- if a map view exists upon adding the control to a map, the default is overriden with the map view projection

(shouldn't a system default projection be defined somewhere in ol.proj.Projection.defaults or the like?)
Creating the map inside the setMap method is a good idea.

I understand how it is less intuitive and agree with you.  Having 'projection' only (for now) is fine.