Initial v0.8 ALPHA commit. Features multi-tasking run-time command execution (feed hold, cycle start, reset, status query). Extensive re-structuring of code for future features.

- ALPHA status. - Multitasking ability with run-time command executions
for real-time control and feedback. - Decelerating feed hold and resume
during operation. - System abort/reset, which immediately kills all
movement and re-initializes grbl. - Re-structured grbl to easily allow
for new features: Status reporting, jogging, backlash compensation. (To
be completed in the following releases.) - Resized TX/RX serial buffers
(32/128 bytes) - Increased planner buffer size to 20 blocks. - Updated
documentation.

nuts_bolts.h

@@ -1,5 +1,5 @@
 /*
-  motion_control.h - cartesian robot controller.
+  nuts_bolts.h - Header file for shared definitions, variables, and functions
   Part of Grbl
 
   Copyright (c) 2009-2011 Simen Svale Skogsrud
@@ -37,6 +37,23 @@
 #define max(a,b) (((a) > (b)) ? (a) : (b))
 #define min(a,b) (((a) < (b)) ? (a) : (b))
 
+// Define system state bit map. Used internally by runtime protocol as runtime command flags.
+// NOTE: The system state is an unsigned 8-bit volatile variable and has a 8 flag limit. The default
+// flags are always false, so the runtime protocol only needs to check for a non-zero state value to 
+// know when there is a runtime command to execute.
+#define BIT_STATUS_REPORT    1 // bit 00000001
+#define BIT_CYCLE_START      2 // bit 00000010
+#define BIT_FEED_HOLD        4 // bit 00000100
+#define BIT_RESET            8 // bit 00001000
+#define BIT_REPLAN_CYCLE    16 // bit 00010000
+// #define                     32 // bit 00100000
+// #define                     64 // bit 01000000
+// #define                    128 // bit 10000000
+
+// Define global system variables
+extern uint8_t sys_abort; // Global system abort flag
+extern volatile uint8_t sys_state; // Global system state variable
+
 // Read a floating point value from a string. Line points to the input buffer, char_counter 
 // is the indexer pointing to the current character of the line, while double_ptr is 
 // a pointer to the result variable. Returns true when it succeeds