CostmapPlugin(ROS) to Layer rename.
This is a big change that renames CostmapPlugin to Layer and folds in the (minimal) functionality
of CostmapPluginROS into Layer as well.  In keeping with this change, I changed all the
plugin names from things like ObstacleCostmapPlugin to ObstacleLayer.
During testing of this I found an issue with the InflationLayer where it did not re-inflate the whole
map when it got a new footprint, so I fixed that as well.
Fixed bug in VoxelLayer::update_bounds() which should save a lot of CPU.
The bounds were being updated with map grid coordinates but they are supposed to use world coordinates.  Map coordinates are much much larger.
Fixed boundary bug in inflation layer.
Added exception handling to _updates topic in rviz plugin.
Renamed update_costs() and update_bounds() to camelCase per coding standard.
Merge pull request #52 from ros-planning/costmap-plugins-renamed-layers

Costmap plugins renamed layers plus bug-fixes