firmware update: features for pronterface use

Changes to be implemented in firmware: BEECONNECT_0.2
M105: Get Extruder Temperature

BEECONNECT_0.1:

    T:11.0000 B:0.0000 ok Q:0

BEECONNECT_0.2:

    ok T:<current> /<target> B:<-237.0> Q:0

Utilmaker,marlin, etc:

    ok T:29.0 /29.0 B:29.5 /29.0 @:0

M110: Set Current Line Number

Example: M110 N123

Set the current line number to 123. Thus the expected next line after this command will be 124.
G90: Set to Absolute Positioning

Example: G90

All coordinates from now on are absolute relative to the origin of the machine. (This is the RepRap default.)
G91: Set to Relative Positioning

Example: G91

All coordinates from now on are relative to the last position.
M302: Allow cold extrudes

This tells the printer to allow movement of the extruder motor, when the hotend is not at printing temperature

Example:

    M302 S<minimun_temperature>

    M302 (assume S0)