Merged and tweaked changes from PR #350 - added multitouch twist support.  General touch control improvements as well.
Fix to HeightFieldUtils to work with a 2x2 hf (aroth)
Controls: fix minimum width bug
Fix: OD bug in which the RTT far plane passed the horizon plane
KML altitude mode fixes
OverlayDecorator fix - empty vertex set caused bad projection
doc updates
Memory fixes for TileIndex driver

Fixed the TileIndex driver so it doesn't need all of the images to be
precomputed.  It can composite as it is collecting now to avoid using
too much memory.
Also disabled the l2 cache on the cached GDAL TileSources to reduce
memory usage.
Merge branch 'master' of https://github.com/gwaldron/osgearth
Removed unused _emptyConfig variable from Config.
Added missing copy constructor to CacheSeed.
Add throwing demo to osgearth_manip
Merge branch 'master' of github.com:gwaldron/osgearth
Add AnnotationNode::getOrCreateAnnotationData
Added MapNodeHelper to osgearth_controls demo
Added a default script engine to Sessions in the event a script is not specified in the style.
Merge branch 'master' of https://github.com/gwaldron/osgearth
Added check in ScriptEngineFactory to prevent repeated attempts at loading a scripting driver that fails.
FileSystemCache: added cache expiration support
Merge branch 'master' of github.com:gwaldron/osgearth