Add vtkMap::ShowArea() to display rectangular area of interest
Update camera on vtkMap::ShowArea(), if renderer has been initialized.

Also:
* Changed code to keep Center coordinates in lat-lon
* Set zoom 1 level lower (undo last iteration)
Merge branch 'master' into john/show-area
Prune conus bounds a bit, so that default display has 1 marker  & 1 cluster
Rename vtkMap::ShowArea() to vtkMap::SetVisibleBounds()
Add GetVisibleBounds() method, to complement SetVisibleBounds()
Merge branch 'master' into john/show-area

Resolved conflicts:
	vtkMap.cxx
Merge pull request #64 from OpenGeoscience/john/show-area

John/show area
vtkMap should register layers when they are added.
vtkFeatureLayer should register features it adds
vtkGeoJSONMapFeature should register PolyData, also delete InputString
vtkOsmLayer should delete cached tiles
Remove memory leaks from qtWeatherStations class
Fix *big* memory leak introduced during vtkOsmLayer refactor

Was adding same map tile twice
Occurred sometime before commit 6149e4c6 (26-Oct-2014)
Fix issues in multithreaded layer, exposed by memory leak fixes.
More memory leak fixes
Replace shallow copy with register call
vtkOsmLayer: refactor std::pow call

Also turn it into a bitshift since it zoomLevel is always an integer,
negative powers don't make logical sense anyways (it's always truncated
to an integer).
Don't need to explicitly delete MapWidget, since it's owned by qtMapFrame
Merge pull request #71 from mathstuf/pow-ambiguity

vtkOsmLayer: replace std::pow(2, int) with a bitshift