Fixed in-line comments and escaping

* "G1 X1 ; test" was not executing "G1 X1" due to never leaving comment mode.
  * "M117 Hello \;)" printed "Hello \" to the display due to not replacing serial_char properly.

Tested with the following commands:
    * M117 Hello ; test => displays "Hello" on display, ignores "test"
    * G1 X1 ; foo => moves 1mm in X, ignores "foo"
    * ; test => completely ignored, not even acknowledged
    * M117 Hello \;) => displays "Hello ;)" on display
    * M117 Hello \\;) => displays "Hello \" on display, ignores ")"

Documentation/GCodes.md

@@ -101,3 +101,25 @@
 *  M908 - Control digital trimpot directly.
 *  M928 - Start SD logging (M928 filename.g) - ended by M29
 *  M999 - Restart after being stopped by error
+
+# Comments
+
+Comments start at a `;` (semicolon) and end with the end of the line:
+
+    N3 T0*57 ; This is a comment
+    N4 G92 E0*67
+    ; So is this
+    N5 G28*22
+
+(example taken from the [RepRap wiki](http://reprap.org/wiki/Gcode#Comments))
+
+If you need to use a literal `;` somewhere (for example within `M117`), you can escape semicolons with a `\`
+(backslash):
+
+     M117 Hello \;)
+
+`\` can also be used to escape `\` itself, if you need a literal `\` in front of a `;`:
+
+    M117 backslash: \\;and a comment
+
+Please note that hosts should strip any comments before sending GCODE to the printer in order to save bandwidth.

Marlin/Marlin_main.cpp

@@ -732,12 +732,15 @@ void get_command()
        serial_char == '\r' ||
        serial_count >= (MAX_CMD_SIZE - 1) )
     {
-      if(!serial_count) { //if empty line
-        comment_mode = false; //for new command
+      // end of line == end of comment
+      comment_mode = false;
+
+      if(!serial_count) {
+        // short cut for empty lines
         return;
       }
       cmdbuffer[bufindw][serial_count] = 0; //terminate string
-      if(!comment_mode){
+
       fromsd[bufindw] = false;
       if(strchr(cmdbuffer[bufindw], 'N') != NULL)
       {
@@ -818,14 +821,14 @@ void get_command()
 
       bufindw = (bufindw + 1)%BUFSIZE;
       buflen += 1;
-      }
+
       serial_count = 0; //clear buffer
     }
     else if(serial_char == '\\') {  //Handle escapes
        
         if(MYSERIAL.available() > 0  && buflen < BUFSIZE) {
             // if we have one more character, copy it over
-            MYSERIAL.read();
+            serial_char = MYSERIAL.read();
             cmdbuffer[bufindw][serial_count++] = serial_char;
         }