Update README.md

Debian wheezy contains packages for gflags and glog.
add the function for emitting SIXEL graphics
add -sixel option
add a function which gets terminal pixel size
fit sixel graphics to terminal pixel size
define sixel RGB palette dinamically
add sixel printer implementation
add new option -sixel2, -sixel16 and -sixel256, insted of -sixel
implement -sixel2/-sixel16 option
Update README
fix a stupid typo
Fix a hanging problem on GNU Screen/tmux
minor fix for processing first pisxel
Fix a bug around controlling color cache
make sure to print right edge pixels
add some comments
prevent emitting redundant sixel control character
Fix for old version of mlterm (<=3.3.1)
update README
Merge pull request #13 from gregf/patch-1

Update README.md