improved the behavior of the log panel in appleseed.studio.

* fix a bug where message coloring broke completely when text was selected.
* don't scroll to the bottom of the log panel if text is selected.
* maintain scroll position if not fully scrolled to the bottom.
* removed redundant std::string -> QString conversions.
Merge branch 'master' of github.com:appleseedhq/appleseed
updated .gitignore.
fixed compilation with gcc < 4.5.
many improvements to runtestsuite.py.

* added a command line option to skip the actual rendering and only generate the HTML report.
* added links to log files in HTML reports.
* delete the output image and the log file prior to rendering a scene.
* check that the output and reference images have identical dimensions.
* optimized image comparison.
project files with multiple entities of the same type and name in a given scope are invalid.
most surface shaders shouldn't override alpha.
base alpha value for surface shaders in a collection is now the same as for the collection surface shader itself.
added Shade Alpha Cutouts boolean parameter on materials.
attempt at fixing compilation on os x.
formatting and other cosmetics changes.
on Linux and OS X, disable console coloring if stderr is not bound to a terminal.
fixed formatting.
added parameters to texture store to enable tracking messages.

new parameters:

   texture_store.track_tile_loading  (boolean, default is false)
   texture_store.track_tile_unloading  (boolean, default is false)
   texture_store.track_tile_cache_size  (boolean, default is false)
renamed texture_store.track_tile_cache_size parameter to texture_store.track_store_size.
fixed Qt runtime warning.
improved log messages.
Fixed OSLBSDF cosine mult bugs.
Merge pull request #308 from est77/OSL_work

Fixed OSLBSDF cosine mult bugs.
added missing #include statements; a few formatting fixes.