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 ol.Map object earlier will create an ol.View if it is not provided. 
The base map view and projection are only known when the control is added to a map, and setMap() is called.

So that would require to delay creation of ol.Map until the control is added and we can safely initialize a View object.