Merge pull request #52 from mike42/feature/35-bmp

BMP format improvements

Merge pull request #41 from mike42/feature/40-wbmp

[WIP] WBMP implementation

Merge pull request #37 from mike42/feature/36-hhvm

Add HHVM to CI

Merge pull request #34 from mike42/feature/17-read-gif

Correct some GIF errors

Merge pull request #33 from mike42/feature/17-read-gif

Implement GIF reader

Merge pull request #32 from mike42/feature/test-coverage

Improve test coverage, related bug fixes

Merge pull request #29 from mike42/feature/19-png

Adds PNG file reader

Merge pull request #26 from mike42/feature/21-docs

Add contributor guide

more updates to adjust project name, closes #24

Merge pull request #18 from mike42/feature/14-indexed-raster

Implement indexed raster image