Added coolant control (M7*, M8, M9). Mist control can be enabled via config.h.

- Added coolant control! Flood control (M8) functions on analog pin 0.
Mist control (M7) is compile-time optional and is on analog pin 1. (Use
only if you have multiple coolants on your system). Based on work by
@openpnp.

- Fixed some variable assignments in spindle control.

coolant_control.h

@@ -0,0 +1,34 @@
+/*
+  coolant_control.h - spindle control methods
+  Part of Grbl
+
+  Copyright (c) 2012 Sungeun K. Jeon
+
+  Grbl is free software: you can redistribute it and/or modify
+  it under the terms of the GNU General Public License as published by
+  the Free Software Foundation, either version 3 of the License, or
+  (at your option) any later version.
+
+  Grbl is distributed in the hope that it will be useful,
+  but WITHOUT ANY WARRANTY; without even the implied warranty of
+  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+  GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License
+  along with Grbl.  If not, see <http://www.gnu.org/licenses/>.
+*/
+
+#ifndef coolant_control_h
+#define coolant_control_h 
+
+#include <avr/io.h>
+
+#define COOLANT_MIST_ENABLE 2
+#define COOLANT_FLOOD_ENABLE 1
+#define COOLANT_DISABLE 0 // Must be zero.
+
+void coolant_init();
+void coolant_stop();
+void coolant_run(uint8_t mode);
+
+#endif