Merge pull request #33 from corngood/develop

Merged fixes of Shader UDLs and Buffer::(Sub)Data
Multiple minor changes
Minor updates in image loading

- Renamed the PNG class to PNGImage.
- Added/moved/removed includes.
Cleanup in image loading/generation

- Moved the implementation of PNG loading to the images/png.ipp file.
- Moved the implementation of images::Checker to the images/checker.ipp file.
Added the XPM2 image format loader
Updated the XPM2 image format loader
Started release-0.39.0
Update of changelog
Merge branch 'release-0.39.0' into develop
Update of web page

- Updated index page and CSS.
- Updated news feed source.
Updated the XPM image format loader

- Added support for non-standard 3D XPM images.
Multiple changes

- Moved certain declarations into separate new files, for more
  lightweight header dependencies.
- Added ifdefs for partial compatibility with GL ES 3.0.
Multiple changes

- Reimplemented the images::FilteredImage template class.
- Renamed the image::Transformed class to image::TransformComponents.
- Added the images::FlipImageAxes image filter.
- Added new constructors to Vector<T, N>.
Cleanup of images::FilteredImage