fix Issue #312
improve palette handling while saving gif and png
reorganized gif optimize to work with palette parameter
replace custom search with dict in gif optimize
be more tolerant of tag read failures

this can occur if the MakerNote has a pointer to outside the EXIF block
Catch truncated zTXt errors.
Python3 fix.
issue warning instead of debug/print
adding prerequisites for tk to the readme
added raspian prereqs
Merge pull request #320 from ericbuehl/master

be more tolerant of tiff tag read failures
Merge pull request #321 from vytisb/master

Fix for #318: Catch truncated zTXt chunk errors when decoding PNG images.
Merge pull request #323 from wiredfool/tk-readme

Include tcl/tk in the readme
Add history [ci skip]
Add history [ci skip]
Merge pull request #315 from d-schmidt/master

fix Issue #312 + gif optimize improvement
Add history
Add respective paths for C_INCLUDE_PATH, LD_RUN_PATH (rpath) to build if specified as environment variables.
Merge pull request #324 from seanupton/master

Add respective paths for C_INCLUDE_PATH, LD_RUN_PATH (rpath) to build if...
Fix bytes which should be bytearray
Catch incorrect palette sizes