Do not convert command to send to upper case after checksum was calculated

Removed for the time being until I have a better idea on how allow stuff like "g28 x0 y0" to be autoconverted to "G28 X0 Y0" but autoconvert something like "m117 Some case sensitive text" only to "M117 Some case sensitive text"

Should fix #103
(cherry picked from commit 628ffb5)
Print 'unknown G/M code' warnings to console once
Merge pull request #122 from norpchen/master

GCode files with unknown commands
Merge branch 'master' into responsive