Two changes to try to achieve repetier firmware compatibility

- Read a "wait" as an empty line in order to send keep alive temperature updates
- Added option to add a checksum to all commands. Needed to add current line tracking for this, let's hope that we'll never get out of synch here...
Merge branch 'devel' into repetier
Reset currentLine when changing state to printing

This keeps gcodePos and current line in sync when changing status to "Printing"
Overhauled resend handling to also work with alwaysSendChecksum feature. Also introduced new feature flag resetLineNumbersWithPrefixedN to make M110 commands send the target line number as part of their N prefix (Repetier), not as a separate N parameter (Marlin & co)
Added known error messages for checksum mismatches or expected line issues from Repetier to recognized "auto-correction" errors, made code around all that a bit more readable.
Fixed off by one error in resend loop
Prevents manual commands from interupting other commands
More reliable initialization of communication with repetier
User thread lock instead of boolean to ensure only one concurrent sending thread, introduced wait flag for repetier printers
Merge branch 'devel' into repetier

Conflicts:
	octoprint/server.py
Removed waitForWait again (that didn't make much sense to begin with...)
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)
Merge branch 'devel' into repetier

Conflicts:
	octoprint/util/comm.py
Merge branch 'devel' into repetier
Print 'unknown G/M code' warnings to console once
Merge pull request #122 from norpchen/master

GCode files with unknown commands
jQuery over SSL and GoogleCDN

Issue #127
Merge pull request #128 from marwue/patch-1

jQuery over SSL and GoogleCDN
Merge branch 'master' into devel
Try to remove timing issues when preparing next line to send