Merge pull request #623 from est77/master

Fixed compile error.
updated Travis build script: the git_tag CMake variable is gone.
Merge branch 'master' of github.com:appleseedhq/appleseed
Fixed foundation <-> Imath matrix conversions
fixed regression in the way bump mapping is computed, and refactored the code.
Merge remote-tracking branch 'upstream/master' into est_imath_convert_fix
Fixed PR comments.
Merge pull request #625 from est77/est_imath_convert_fix

Fixed foundation <-> Imath matrix conversions
fixed compilation on Windows (signed/unsigned mismatches).
removed unused local variable.
updated Hosek-Wilkie day sky model data file with latest version.
python bindings improvements
Merge pull request #626 from est77/est_python_update

Python bindings improvements
Enable building python bindings in travis.
Added some debug output
Added some debug output
Default to python 2.7 if version string not defined
Default to python 2.7 if version string not defined
Update unaligned_matrix44.h

Replaced int by size_t in for loops.
Merge pull request #627 from est77/est_travis_python

Enable building python bindings in travis.